There are several ways to UV unwrap and they are all for technical reasons.
As you mentioned, the head might be separate so that it gets a different (larger) image than the rest of the body.
Another way is to have the head on the same texture file, but occupying a larger space.
You can also UV half the mesh with the other half mirrored and "folded under". This will look like you textured half the body, but it covers both sides. This is mainly to conserve resources, as with background characters or with realtime (videogame) characters.
You need to create seams to unwrap. To separate the head, you usually create a seam that goes all the way around the neck. Where the seams go, however, depends a lot on what the geometry looks like.
Once you unwrap, assign the textures. Mind that the texture you see is just for the preview, it wont show up on the render until you assign materials.