According to wikipedia, the term "post-truth" originates from a 2010 article on Grist: Post-Truth Politics. According to the author, the Enlightenment (pre-post-truth) idea of voting is that each voter gathers facts to reach a conclusion about political issues, forms political opinions based on that work, and then selects a party or candidate which matches their political opinios.
In the post-truth scenario, voters choose a party (or a "tribe") based on personal values. They adopt the political positions of their group, then develop arguments to support their position. These arguments carefully screen out facts which do not support their argument.
Normative vs Descriptive Statements
The question references the ideas of normative and descriptive statements (or questions). In both cases, voters are asked about both normative ("which candidate should I vote for?") and descriptive ("how many people actually attended the inauguration of Donald Trump?*") questions.
The difference is the order of operations: in the truth scenario, political positions are built on evidence and rational thought. In the post-truth scenario, facts and conclusions are molded to fit the argument at hand. Below I use the recent discussion of inauguration attendance as an example of both scenarios.
In the "truth" scenario, people build their normative conclusions on factual decisions. For example, if I conclude that Presiden-Elect Trump's inauguration was not the best attended in history, I may be skeptical of his claims and be less favorable toward him. My political decision (how much I support President Trump) is based on a factual appraisal of how many people attended the inauguration. If the evidence showed that he was correct, I would have evaluated him more favorably.
In the post-truth scenario, people choose the normative positions first, and then tailor their arguments and selection of facts to fit their normative position. Example: If I am committed to President Trump's viewpoint and agree with my party, than I am not open to evidence or argument suggesting that the inauguration wasn't the biggest in history. Or, I may change the frame of the question to something that fits my preconception. No evidence will change my mind.
Some Other Descriptions of Post-Truth
Wikipedia presents some other descriptions of post-truth politics:
- "campaigners continue to repeat their talking points, even if these are found to be untrue by the media or independent experts"
- conspiracy theories may become major news topics
- A situation where public opinion is disconnected from the factual nature of policy
*I'm using the inauguration attendance discussion as an example, not to imply what the facts on either side of the issue are. If you want to discuss the facts try any of the bajillion posts on Skeptics.SE