"Pretty" is less extreme than "very." For example: If an object is very heavy, it is difficult to lift, maybe impossible. If it is pretty heavy, it is possible to lift.

They have the same meaning. But "pretty" is more casual. Also, "pretty" is not quite as intense as "very" If you took a test, and got an 85%, you did "pretty good." If you took a test, and got a 100%, you did "very good."

You can also use "pretty" as an adjective.