What are the battle tents like?

You use rental PKMN in Battle Tents.

The Battle tents are variations of battle tournaments that aren't much like the classic Battle Tower. Slateport has Battle Swap, in which you start with three rented Pok�mon and swap with a defeated opponent between turns. In Verdanturf, you let your Pok�mon battle on their own accord. In Fallarbor, you play Set KO Tourney, in which you have to knock out the opponent in three turns, after which the winner will be decided by evaluating the remainder of HP, the number of successful attacks, and the Pok�mon's aggressiveness. Pok�mon Swap can later be played in the Battle Factory in the Battle Frontier, the Verdanturf challenge will be hosted in the Battle Palace, and Set KO Tourney will be hosted in the Battle Arena.

The battle tents are where the contests were in Ruby/Sapphire. You just go in them and battle people.

Adding to what others have said before me, yes - there is a place like the TRAINER tower in ruby/sapphire. These are not the battle tents, however. When you get to Battle Frontier (shortly after Elite4), go to the BATTLE tower. This is very much like in ruby/sapphire.

There is also a TRAINER HILL, which can be found above the town with the original game corner and the electric gym leader [can't remember name of town]. The Trainer Hill is very much like the Trainer Tower in FireRed/LeafGreen.

Hope I helped! : )