what is a ginger? I have mostly heard it in reference to red heads, but what does it mean?

It's just a slang word for someone with freckles, paler skin, and has red/blonde-ish hair. (mostly red) It's immature though, and if someone refers to someone as a "ginger" they're not using it as a nice term. There are many jokes along with this word as-well.

Ginger looks like peanut butter colour :)

It's a comparison of color. If you look at actual ground ginger it's a sort of orange-ish color.