SSRI = selective serotonin reuptake inhibitor.

Couple of concepts here. The several classes of antidepressant work by
increasing the amount and/or time that one or two neurotransmitters can
work on a receiving nerve cell (neuron).  There are differences in the
way they work in the gap (synapse) between the transmitting and
receiving nerve cells.  The two neurotransmitters of general concern are
serotonin and norepinephrine.

Reuptake inhibitors work on the transmitting cell, which senses how much
neurotransmitter it has released, and "sucks back" (reuptake) the
excess.  Other classes decrease the effect of an enzyme in the receiving
cells, an enzyme that breaks down the neurotransmitter. By decreasing
that effect, the neurotransmitter acts longer on the surface of the
receiving cells.

So, a quick summary of the major classes, without getting into
variations within a class or atypical drugs:

Tricyclic antidepressants (TCA): Postsynaptic. Increase both serotonin
and norepinephrine activity by inhibiting the enzyme catechol-O-methyl
transferase (mercifully, COMT)

MAO inhibitors: Postsynaptic. Increase both neurotransmitters by
inhibiting the enzyme monoamine oxidase. Rarely used because they have
extensive and dangerous reactions with a wide range of drugs and food.
Why fix a depression and find out you can't eat chocolate or drink
Chianti?

SSRIs: Presynaptic, inhibiting the reabsorption of serotonin alone.

Effexor and a few other drugs without a real class name: Presynaptic,
but inhibiting the reabsorption of both serotonin and norepinephrine.

There is at least one selective norepinephrine reuptake inhibitor, but I
can't remember the name.

Incidentally, direct stimulants like amphetamine and Ritalin are
sometimes appropriate, especially for childhood or sometimes geriatric
depression. Lithium and anticonvulsants are useful for a bipolar
disorder, with mood swings from super-high to depressed, or for manic
mood alone.

Also, there can be excellent reasons to use more than one
antidepressant, or an antidepressant with an anti-anxiety drug, or other
combinations.