Each turn consists of two phases: the Defense phase and the Main phase.
During your Defense phase, you can convert cards to energy, play most action cards (with a few exceptions like Slime Rain), play cards from your bench, and defend against your opponent's attacking allies.
During your Main phase, you can still convert cards, but you can no longer defend-- all the allies you didn't defend against have dealt their damage, and are exhausted. You can play or bench a card of any of the four subtypes: ally, trap, object, or action, as well as attack and use any special ally abilities.
Here's what the rules on Orbs CCG say:
"At the beginning of your Defense Phase, your energy regenerates and you draw a card. During the Defense Phase, you can do one of four things: defend against any attacking allies, discard a card for energy, use the ability of a card in play, or play an action card from your hand. Note that you cannot play allies or objects during your Defense Phase.
At the end of your Defense Phase, you take damage from any undefended attacking allies equal to their attack points (AP).
At the beginning of your Main Phase, after your Defense Phase, all exhausted allies you control become unexhausted. During your Main Phase, you can play any card, use any ability, and declare attackers.
When you are done with your Main Phase, click the "End Turn →" button on the right-hand side column. It'll then become your opponent's turn."