I am not going to describe the gametype again the other solutions above covered that perfectly.
To win these matches, good teamwork and communication would be great, but if people aren't communicating, then figure out your role on the team. For example, when playing on Gridlock, one round I was the "ring bearer", staying in the ring while the others did everything they could to prevent the other team from getting control. And I of course also protected the ring when threatened. We actually shut them out one round. We fell into roles naturally as per what was needed. If I died, I'd become the defense or the grenade planter, etc.
An update to one of the previous posts: there is now a King of the Hill playlist that you can go to so you don't have to worry about this gametype not being selected by the majority of the players anymore. The players in that playlist for the most part are quite experienced, so you should have no problem getting on a team that works well. I got my 3 wins in 5 games, no problem.