The reaction follows the normal rules for an acid-carbonate reaction, which in turn is a special kind of acid-base reaction.
Acid + Carbonate -> Salt + Water + CO2
(Citric Acid) + (bicarbonate soda) -> (Trisodium citrate) + water + CO2
This is a very fast reaction, which produces a lot of carbon dixoide in a very short amount of time, which is the bubbles that you see. The other chemicals produce are clear liquids, so you don't see any of them.