There is only one kind of Serotonin.
In depression certain neurotransmitters are relatively lacking.
One of those is serotonin. The SSRIs (Selective Serotonin Re-uptake Inhibitors) slow down the process of returning the serotonin to the end of the neuron it comes from. This leads to the chemical remaining in the vicinity of the receptors for longer, making it more likely that enough will build up to set off the impulse in the next neuron.
Thus, the SSRIs work by allowing the body to make the best use of the reduced amounts of serotonin that it has at the time. In due course, the levels of natural serotonin will rise again, and the SSRI can be reduced and withdrawn.