Sun: Simulate a distant light source that behaves like the Sun.
Spot: A point that cast light in a cone shape.
Area: A squared shape that emits lights from all the available
shapes.
Each one of those lights has a purpose in a project. For instance, in a
scene that tries to simulate daylight, you will probably use a Sun
combined with an HDR map for a Cycles render.
It is possible to add each new lights using the SHIFT+A key or change
an existing light source type by selecting it and opening the Object Data
tab. There you will see buttons at the top where you can quickly swap
between each type of light (Figure 6.22).