Unless the weather has been particularly insane, people just don't wear hats anymore. Unless you're in the Tundra or at the beach, not too many people are out for a walk wearing a hat. The exception to the hat decline? Baseball caps. Not just for America's past-time, baseball caps can be worn in support of any sports team, or just for the heck of it.
Style
While baseball caps are fairly straightlike crown to them, where longer and rounder brim caps look more like ducks, with a low crown.
Fabric is another consideration. Anything that's going to be on your head for long periods of time should be breathable and flexible, so look for a cotton or cotton blend cap.
Size
While other hats are specifically sized, most baseball caps are either Adult or Children's sized, then divided into small, medium or large. This means you can have trouble finding a good fit if you're head isn't a perfect Adult Medium. There are Velcro and slotted straps on most caps, that adjust for a better fit. The downside is the adjustable straps reduce skin coverage and are painful for those of us who wear pony tails.
Speaking of skin coverage, a baseball hat is no substitute for a sun hat or SPF. Even the widest brim baseball cap leaves the sides of the head and neck exposed, even if it does keep the sun from your eyes.
Team Spirit
The most popular reason to wear a baseball cap? Team spirit, of course! If there's a sports team, there's a hat for it. You can also wear caps that advertise your love of certain brands and companies, or buy blank baseball caps that you can decorate yourself, for Little League or as a wedding party gift.