Simply put, what exactly is the whole Ukraine crisis about? I've read things about it, but I haven't really found anything concise and clear.
Also, how/why is the U.S. involved?
What it's about:
The question is really too broad to be answered properly, but let's try to boil it down to bumper stickers:
Russia has a desire to control neighboring territory (directly or indirectly).
Reasons vary (defense depth/military; economics; energy; national psyche)
Ukraine is a prime target in ALL of these reasons:
It is a defensive buffer from the southwest; it offers Black Sea access via Crimea; it has gas line to Europe, it has its own energy resources (coal/oil/gas); it was owned by Russia in USSR days; tons of population in some regions are ethnically russian; tons of other population hate Russia and Russians and Russian control.
What's going on
Russia tried to get Russia-friendly leadership once via election fraud in 2004.
That failed in what became known as Orange Revolution
Russia managed to get Russia-friendly leader (Yanukovich) elected again, in 2010
Yanukovich chose to sign an agreement about economic integration with Russia, contrary to the wishes of the large fraction of population who wanted a competing integration with EU.
The latter organized protests known as EuroMaidan
Yanukovich suppressed the protests, with very brutal methods including killing of protestors
He was forced from power as a result and fled the country, back to his masters in Russia.
Russia invaded and forcefully annexed Crimean peninsula, using unacknowledged special forces.
Historically, used to be Crimean Tatar territory, taken over by Russia; then Tatars were genocided out and it became largely Russian demographically; then was given to Ukraine by one of the USSR leaders.
Russia then sent its special forces to pretend to be separatists in Russian-demographics-dominated areas of Eastern Ukraine
This launched various insurrections, armed rebellion and such.
Sources (not linked above):
It is about two totally different issues that have started to interact with each other
As far as the first issue is concerned: Ukraine has been a politically divided country since independence after the end of the Cold War. There is a pro-European faction and a pro-Russian faction, they have been unable to put their differences aside and make decisions that are good for the country. Ukraine's economy has stagnated for a long time, Ukraine has had corrupt leaders (from both factions) making things even worse.
he latest flare up between the two factions was about whether to sign the EU Association Agreement or to sign the Customs Union with Russia. The refusal of the Ukrainain government to sign the EU Association Agreement led to the protests and the subsequent violence led to the overthrow of the government, president Yanukovich fled Ukraine.
These events led to a dispute between Russia and the West. The West accused Russia of exerting inappropriate pressure on Ukraine to not sign the EU Association Agreement while Russia accused the West of supporting the demonstrators against the government 9the Euro Maidan movement) who according to the Russians did not represent all of Ukraine.
When the government was overthrown and replaced by a government that was willing to sign the EU Association Agreement, Russia considered that as an illegal coup. The West raised the fact that Yanukovich was corrupt and had illegally ordered violence against the demostrators (with facts uncovered since he was forced from power to back that up), while Russia mantained that even if true, these issues should have been dealt with the existing constitutional mechanisms (e.g. the impeachment procedure).
From then onward you get an interaction with this dispute with the second issue of the West losing power and Russia exerting itself more in the international arena. After the end of the Cold War, Russia hada pro-Western government under the Yelstin administration. There was some disagreements, e.g. about the former Yugoslavia and Chechnya, but overall Russia was far more pro-Western than it is today. However, the economy was in a big mess and the oligarchs had gained a lot of wealth which led to the public becoming more and more sceptical of Western values that was supposed to bring them economic prosperity.
When Putin came to power, he turned the economy around using the oil and gas revenues. His way of ruling the country was more authoritarian, less democratic which led to criticism from the West. That criticism combined with the public perception that things were improving under Putin led to the public that was already sceptical of Western values to become more anti-Western.
On the international stage Russia has been in disagreement with the West on issues such as the Iraq war, the Iranian nuclear issue, the war between Russia and Georgia, the West's recognition of Kosovo's independence, the way NATO intervened in Libya, and the civil war in Syria. On some of these issues, Russia was brought on board the preferred Western policy. E.g. in case of Iran's nuclear program, the US did a deal with Russia where Russia would not veto a sanctions resolution on Iran for Iran's refuals to halt enrichment (despite Russia's previously stated position that Iran as a sovereing country and IAEA member has the right to enrich uranium), in exchange for the US not imposing sanctions on Russian companies involved in the Bushehr nuclear reactor in Iran.
Russia's overall perspective of the West is that the West behaves like the World's dictator. The West has so much power that it can build a coalition against Iraq or Iran based on flimsy evidence (countries can be de-facto bought to support the Western position), impose biting sanctions or even start a war and when the original issue the West made is proven to be wrong, the West is not going to be held responsible for anything.
In case of Libya, when NATO had the UNSC mandate to police the fighting between Gaddafi loyalists and the rebels and to police the weapons embargo on both sides, NATO gradually deviated from what it was allowed to do to becoming the rebels' air force and they facilitated arms deliveries to the rebels. While the West may have had good arguments to do this, it was still illegal under international law to do that without a new UNSC mandate. However, the West maintains it was legal to act in that way, which has further undermined the little remaining trust Russia had in the Western interpretation of international law. So, to Russia, what the West says about conflicts that the West is istelf involved in, is not to be trusted at all.
Then back to Ukraine after the overthrow of the government there. The dispute about this in the above context would never lead to fruitful discussions in which both sides would get agreement. Russia's persective was that now that the West was asserting itself in Ukraine, it would be pointless to support the rights of the pro-Russians in Ukraine while sticking to the fine print of the constitution.
Crimea becoming part of Russia has always been an issue most Russians wanted to see happen eventually, but with Ukraine moving toward Europe and the West supporting actions that are not consitutional, Russia had no faith that this issue could ever get fair hearing. If a pro-Western faction per se does not want it, the West would support strong arm tacticts to prevent it regardless of what the Ukrainian constitution says. To the contrary, Russia feared that the parts of the country that were not so sympathetic to the new authorities in Kiev would be subject to armed intervention.
These fears may not have been all that reasonable, but this led to the Russian military intervention in Crimea and the subsequent intervention in Eastern Ukraine in support of the rebels there. The Western criticism of Russia's actions then makes Russia even more determined to persue its present course. Simply put, when the Ukrainian army is shelling Donetsk, civilians are getting killed there, and Russia is coming to the aid of the rebels, the West condems Russia. To Russia this doesn't make sense, so they ignore the Western criticisms of their actions.
Legalistic reasonings based on international law are going to be ignored because Russia sees the West as having been wrong numerous times in the past and Russia obviously does not accept the West as having the standing to make binding judgements on what it is allowed to do, even if the West were right.
Then the West has not come to terms with the fact that they are not as powerful as they used to be, so they think that they can force Russia to do the right thing by imposing more sanctions. But as explained above, this amounts to adding fuel to the fire, so we should brace ourselves for a big conflict between Russia and the West.
The conflict in Ukraine, just like the vast majority of other international conflicts, is about resources. The last vast reserve of the Earth's untapped natural resources happens to be in Russia. The West needs those resources to subsidize its shop-till-you-drop consumerist lifestyle.
Putin is the West's "bad" guy #1 because he put an end to the fire sale of Russia's natural resources and the depopulation of Russia's territory that took place in the 1990s under Yeltsin and his US advisors.
Given that the Iraq option is not available due to Russia's nuclear arsenal, the West is trying a different tack. They bribed/blackmailed a faction of Ukraine's oligarchs (many of whom keep their loot in the Western banks) to support the West's goals. Two years ago, the pro-Western oligarchs staged a coup d'état against the pro-Russian oligarchs then in power in Kiev.
The West's plan is to put a gun to Russia's head (in the form of NATOs missiles and troops in Ukraine) either to force Putin to "cooperate" or to convince a faction of the Russian oligarchs/elites to overthrow Putin.