What is Jazz Improvisation?

The thing that I think attracts musicians and listeners to jazz the most is the improvisational aspect. Jazz, as it has been practiced in this century, allows great freedom to the musician to "make up" his/her own solo. (This used to be a trait of classical music, but in this century has largely been performed according to precise musical notation.) But how much of jazz music is improvised and how much is learned material? Most who have studied jazz improvisation even a little bit understand that large groupings of notes (licks) can be found repeated in just about all of the great musician's recordings, in fact an artist's vocabulary is one of the traits that defines him/her as an original. Much of what is improvised is the order in which material is presented, how it is inflected, where it is placed on the beat, and how the other muscians interact with it. I know that when I first started learning licks, fellow students would admonish me that this would turn me into a "lick player" and not a "true" artist. But I have discovered the opposite to be true. The artists that I most admire have learned and digested great volumes of other musicians vocabulary and have subsequently reinvented these ideas in their own way and thus distinguished themselves as original artists.