2D animation is what is normally termed "traditional," since it comes from a family line including the Disney classics and Warner's "Looney Tunes." Each tiny movement of on-screen characters is drawn by hand and a feature-length film can have hundreds of thousands of drawings. Though computers have been used by Disney as an aid to 2D animation since at least 1989, computers have primarily been used for inking and coloring support, while the major workload of creating the individual drawings is still done painstakingly by hand.
On the desktop, there are a number of 2D animation aids available, though most are commercial projects and tend to be expensive. One formerly-commercial product has made the switch to being an open source project and now is available to all comers free of charge.
SynFig claims to be designed from the ground up as an animator's tool and to follow an animator's workflow. If you're into animation, give it a look, as it promises to be a real time-saver.
Synfig - Overview
