Kippology

You don't actually have to wear a kippa, even if you want to cover your head: any other head covering will do. But a kippa has become a symbol as well and, weighing less than a hat or cap, is so comfortable you hardly notice you are wearing it.

Wearing a kippa indoors is also more easily accepted than a cap, which in some Jewish schools is even forbidden for indoor use.
As for its symbolic value; any kippologist will be able to tell you a lot about the wearer of a particular kippa. It's a fine-tuned indication of the identity - Jewish or otherwise - of the wearer.

You can wear one with the logo of your favourite sports club or cartoon character - though the latter is off-limits in some Jewish schools. You can wear a school kippa though; in the UK particularly the kippa is part of the school uniform. Boys with a bit of guts love to wear an IDF kipa.

The religious flavour of the wearer can also be 'computed' from his kippa. A good rule-of-thumb is: the bigger the kippa, the frummer the wearer - but that's not all. Some Haredi sects are referred to by the name kipot shkhorot (Hebrew כיפות שחורות), literally "black kippot". Kippot shkhorot are usually sewn of black fabric and often larger than the ordinary ones. Modern Orthodox Jews typically wear either black leather kippot or knitted -or rather crocheted- kippot. In the non-black range, the chrocheted kippa is a bit more versatile. The Israeli Religious Zionist community is often referred to by the name kippot srugot (Hebrew כיפות סרוגות), literally "knitted kippot" - but these are typically larger than usual and you won't catch them wearing a really colourful one.

The middle-of-the road person will wear crocheted, suede or fabric kippot in whatever look takes their fancy, and in a size neither too big or too small. (If your kippa is too small that could be taken as a statement, rather than a sign you were in a hurry and grabbed one of your little brother's or son's... You'll find  a handy kippa size chart here.) These come in all colours and patterns; you could find a matching kippa for any shirt. Usually the colouring is not too flamboyant but if you want to look special there is a wide choice, including some great-looking sefardi kippot.
And then, of course there's the kippa you can get at a bar mitzvah or wedding if you're lucky - but those are a whole new chapter.