Composite nodes define the root of a branch and the base rules for how that branch is executed. They can have Decorators applied to them to modify entry into their branch, or even cancel out mid-execution. Also, they can have Services attached to them that will only be active if the children of the Composite are being executed. 

Only Composite nodes can be attached to the Root node of a Behavior Tree.

Selector

Selector nodes execute their children from left to right. They stop executing when one of their children succeeds. If a Selector's child succeeds, the Selector succeeds. If all the Selector's children fail, the Selector fails.

Sequence

Sequence nodes execute their children from left to right. They stop executing when one of their children fails. If a child fails, then the Sequence fails. If all the Sequence's children succeed, then the Sequence succeeds.

Simple Parallel

The Simple Parallel node allows a single main Task node to be executed alongside of a full tree. When the main Task finishes, the setting in Finish Mode dictates if the node should finish immediately, aborting the secondary tree, or if it should delay for the secondary tree to finish.