There's a great difference! The cello is smaller than the double bass, but it's got a beatiful sound. Think of a violin or viola but deeper but with the same "weeping" tone. Look up this song for a great example of cello:
Johann Sebastain Bach: Six Unattended Cello Suites: Suite 1, part 1: Prelude (BWV 1007)
There are lots of examples of the sound of a cello, but this song is one of my favorite cello pieces.
The double bass, however, is the instrument that you see those old big band jazz bass players play. They're those big instruments and have the deep and rumbling sound to them. I've never played them with a bow, but playing pizzacatto (finger style) on them produces a great, strong sound that'll shake the ground! I've got no songs for examples... If you can, go to your a Sam Ash or something (the Sam Ash near me has a orchestral instruments section) and just play a few notes on the double bass! That's power, baby.
I know I haven't given you much descriptions of their sound - I'm no genius with words. Maybe someone can come along and clear it up. But here's my point: The cello and double bass are wonderful instruments. You should try them out one time!