Soldering: the new chip itself (or piggy back board) is soldered onto the board.
Socketing: the "old" chip is removed and a socket soldered to the board in its place.
You can then plug the "new" chip in, and remove it to plug the "old" chip back in if needed.
Usually the new chip has a small piggy back board it is attached too.
Whatever you do, don't do it yourself or have a friend do it. I had a guy do mine that solders chips on satelite communications gear for a living and I ended up buying a new ecu. Send it to an authorized dealer. I can't say enough good things about Dubwerks in Libertyville IL. Rich is awesome.