Which component, and when

One short entry per component: what it is for, when to use it instead of the one next to it, and which states it needs. It answers which one do I use, not how do I build one. It is meant to be searched, not read.

No entry is written yet. The full index is below anyway, because the shape of what is coming should not be hidden: 35 components, each of which will get one page.