I've had experience in both a high school and college marching band and while the college band is vastly more enjoyable because of organization and dedication of the members, there isn't much difference in the general feel of it. It's much different from concert band where the emphasis is on balance, tone, and musicality. Marching band is all about being as loud as possible and making the music crazy and fun, even at the expense of quality. To achieve volume marching bands use lots of brass instruments. Some bands, college bands especially, sometimes do not allow flutists or, more commonly, clarinetists (like myself) to join. However, if the band is on the small, member-desperate side you are more likely to see some woodwinds.
The most basic duty of a marching band is to march a field show at half time of football games. During the show the band plays while using its members to form designs on the field, marching in and out of different shapes. The band may also march in formation during parades. Also, there is often a flag line or color guard. The good ones are amazing and add a lot to a show. Besides marching, some bands also play pep band at football games and other sporting events. However, in some cases the pep band is a separate organization.
Even though generally not as "cool" as the football players or the cheerleaders, a marching band member sports his or her unstylish uniform with pride and probably has the most fun of anyone at a football game. Band practice and performances are always loud and full of goofing off. Social activities, such as trips and the infamous band camp, generally involve quite a bit of alcohol.
The music played varies from serious drum corp stuff to the ever-present Blues Brothers music to often corny profound pieces. Most of the time, marching band is not a place to be profound.