Purpose is thoughtful and deliberate goal-directedness -- thus "intentional" behavior. Intention to commit a crime is sometimes a crime by itself, see grooming a child.
In the philosophy of mind, intentionality is "aboutness", i.e. an intentional state of mind is one that is "about" something -- i.e. has some subject matter.
Intension (with an s) is a different word easily mistaken for intention (with a t).
