Anxiety disorders cause excessive fear and worrying and can even make people sick. Symptoms can be severe enough to make it hard for people to function.
Bipolar Disorder causes severe mood swings and depressive/manic episodes.
PTSD, or Post-Traumatic-Stress-Disorder, is a disorder commonly developed after traumatizing events. It causes severe anxiety and nightmares.
Autism is a disorder that effects the development of the brain, specifically in social areas.