Aikido is Japanese and is based around jointlocks and projections. Hapkido is Korean and has lots of striking and a little bit of jointlocking in it. col
The main diffrence between Aikido and Hapkido lies in the redirection of the attacker. The Hapkido man counterattacks so strongly and swiftly with tremendous force that it totally overwhelms his adversary. Whereas in Aikido, the main principle is to just quell the attack, without force. A Hapkido man will never meet an attack head on. Insted he will step to the side before counterattacking. Kick, strikes and pressure points are a big focus too. There is no katas in Hapkido. I'm doing some research into both of these styles as Hapkido is a style i'd like to take up - along with Shoulin Gung Fu - after i reach 1st Dan in Tang Soo Do.
There are at two other threads on aikido vs. hapkido. They will probably answer your question, Anomale.