Selective Serotonin Reuptake inhibitors such as Celexa and Prozac are a class of drugs that
inhibit the uptake of serotonin by the central nervous system or (CNS). These type of antidepressant that acts by blocking the reuptake of serotonin so that more serotonin is available to act on receptors in the brain.
With Effexor, which is a MAOI Monoamine Oxidase Inhibitors. MAOIs work by inhibiting the activity of monoamine oxidase, preventing the breakdown of the brain’s monoamine neurotransmitters. This effect is thought to increase the available stores of serotonin, norepinephrine and dopamine. When these excess neurotransmitters are not broken down and absorbed, they begin to build up in the brain. Since depression may be associated with low stores of monoamine neurotransmitters, increasing them may help to alleviate depression disorders.
Here is some links so you can read up on them.
http://en.wikipedia.org/wiki/MAOI
http://en.wikipedia.org/wiki/Selective_serotonin_reuptake_inhibitor
http://en.wikipedia.org/wiki/SSNRI