Depression generally takes one of two major forms. Unipolar depression is what most people mean when they talk about depression: an unremitting state of sadness, apathy, or hopelessness, and loss of energy. It is sometimes called major depression. Bipolar depression, or bipolar disorder, is a condition marked by periods of depression and periods of high-energy mania; people swing between the two poles of mood states, sometimes over the course of days, sometimes over years, often with stable periods in between.
The birth of a baby can trigger mood swings or crying spells in the following days or weeks, the so-called baby blues. When the reaction is more severe and prolonged, it is considered postpartum depression, a condition requiring treatment because it can interfere with the ability to care for the newborn. Depression can also occur seasonally, primarily in the winter months when sunlight is in short supply. Known as seasonal affective disorder, or SAD, it is often ameliorated by daily exposure to specific types of artifical light.
Sometimes depression manifests as a persistent low mood, a condition known as dysthymia. It is usually marked by years-long periods of low energy, low self-esteem, and little ability to experience pleasure.
Suicidal thoughts often coincide with depressive episodes, which is why it's important to be aware of the signs if you or a loved one experiences any prolonged mood disturbance.