For example, I don't know what the major differences are between the Greens and the SPD. On a crude left to right line, where would you place the major parties? Comparing them to US parties, where would you place them?
Compared to US parties, all of them with the possible exception of the AfD are leftist. Most favor public healthcare for all, for instance, and a government role in the pension system.
The German system of proportional representation with a cutoff encourages large and medium parties, not very small ones. A party which consistently scores below 5% is likely going to dissipate. A new party breaking above the 5% level happens perhaps once in a generation.
From left to right, and trying to force the federal and sixteen state organizations of each into a single paragraph:
- Die Linke
Literally "the left," they are in favor of legislation aiding poor workers. They also have some political baggage from being the successor of the Communist party in East Germany. For instance, they are anti-NATO and to some degree pro-Russia.
- Die Grünen
Formed out of environmentalist and peace movements in the West, and the democracy movement in the East. They have grown from a special-interest, "niche" party into being the senior partner in state-level governing coalitions. Still quite environmentalist. Their leftist bend somewhat conflicts with the attitudes of wealthy environmentalists who can afford to buy organic meat, etc. and expect others to do the same.
Social democrats. Originally labour, now with many supporters in the civil service and similar jobs. Once they were one of the big, non-special-interest parties, they are much diminished in recent years. In parts this is due to being the junior partner of the Union for the last decade. They're being ground up between die Linke and the Union.
Liberal democrats. Another "niche" party supporting tax breaks and freedom of business for small businessmen, etc.
- CDU and CSU (together the Union)
The CSU in Bavaria and the CDU in the rest of Germany call themselves Christian Democrats. They are more conservative and more likely to be religious than the other democrat parties, but they are the other of the big non-special-issue parties, covering a broad platform. Some of their workers' advocates have been nicknamed "Sacred Heart Marxists" for their faith-inspired support of government welfare. Others are "nationalist conservatives."
The Alternative for Germany developed from an anti-Euro (and especially anti-bailout) platform over anti-immigrant policies to an authoritarian worldview. Many have pro-Russia and conspiracy-theorist worldviews, and a vocal faction are fascists.
Historically, there has almost always been a coalition government. This would be either the SPD or the Union as the senior partner, with one or perhaps two smaller ones to get the necessary majority. This has broken down in recent years when the Union and SPD formed a "grand" coaltion which has become increasingly less grand over time.
On the federal level, the Greens, SPD, FDP and Union have been involved in governments. At the state level, die Linke were included as well. All other significant parties are unwilling to work with the AfD, but eastern CDU state-level parties are wavering in this regard.