PT() is an automatic pointer for objects that inherit from ReferenceCount. It automatically increases the reference count when you create one and it decreases it when it goes out of scope, making sure that objects automatically get cleaned up when there are no references to them anymore. You should use them to refer to reference counted objects (including PandaNode derivatives). Be sure not to attempt to delete objects that inherit from ReferenceCount.
DCAST is a macro for objects that inherit from TypedObject that performs safe typecasting.