Selective serotonin reuptake inhibitors (SSRIs) and serotonin-norepinephrine reuptake inhibitors (SNRIs) are antidepressants. They are used primarily for anxiety disorders.
SNRIs seem to be more effective than SSRIs, but they may have more side effects that some find difficult. SSRIs are likely used as the first-line of defense against an anxiety disorder.
SSRIs are selective and increase serotonin in the brain while SNRIs increase both serotonin and norepinephrine.