Basically, squads are groups of players who have decided to fly together in some sort of structured fashion.
Squads vary wildly in their attitudes, expections, goals, and structures. Some are very laid back, non historical, and basically just a group of guys who fly together when they are playing. Other squads are very historically minded, recreating an actual WWII squadron, flying dedicated squad aircrafts (i.e. a P-38 squad, or BF109 squad, etc), flying structured missions, formation flying, wingman tactics, etc. And there are any number of squads who fall somewhere in between.
Most squads, even the highly structed ones, only will require you to fly specific types of aircrafts, or participate in structured missions/events, on designated nights, or for specific special events, and you would be free to fly whatever/whereever you wanted at any other times.
In my opinion, you should ask yourself what you are looking for out of a squad(i.e. types of planes, historically minded or not, what nights you can participate in their squad nights, what special events you'd like to participate in, etc.), and look for a squad that you think you would naturally fit in with. There are tons of great squads and players to hook up with in this game, and they all offer something a little different from the next.
Hope this helps. Good luck to you. In my opinion, being part of a squad adds an extra dimension to the game, and a lot fun too.
