Raichu is the evolved form of Pikachu from the original Pokemon games. Even though the anime made you feel bad for evolving your Pikachu, I did so with no regrets.
Speaking of the anime, that episode with Lt. Surge's Raichu was great. Ash's Pikachu was no match until wait... a little training. Eh I'm just going to let my pokemon evolve.
They kept Raichu's name the same in the English versions from the Japanese original. The "Rai" comes from 雷 (Rai) a Japanese word for thunder. While "Chu" comes from チューチュー (chuchu) a Japanese onomatopoeia for squeaking.