Is this game teamwork centered, or isn't it - does one need to capture a flag or something like that?
Battles are won by capturing or defending objectives, which requires your team to have more people than the enemy around the objective. It can require a lot of teamwork to do this, especially if you need to deal with tanks and obstacles as well as infantry and machine guns. You can't do it all yourself.

Is this game infantry centered, or does this simply depend on the map one plays? How realistic is the infantry combat ?
The game is more infantry centered than vehicle, but that is largely map-dependent. There are three kinds of maps - infantry-only, tank-only, and combined arms. The first two are self-explanatory, but in CA maps, infantry tend to be more important than tanks, that is, objectives can be accomplished in spite of the best efforts of tankers. Which makes sense. Tank support makes it much easier for ground-pounders, though.

The combat itself, while limited in realism (it is a computer game) is the most accurate game available except Armed Assault or Operation Flashpoint. It's in some ways similar to those games.