Domain mapping allows you to buy a domain and have it show content that is hosted someplace else. Basically, you’d buy a domain and then map it to your BlogSpot account and it’d appear that you’d have a fully hosted website. No one would see http://you.blogspot.com anymore, they’d see http://www.cool-domain.com.
The plus side to this is that you don’t need to buy hosting. Just keep using Bloggers webspace and resources. All you’d need to do is buy a domain, usually $9.99 a year, and work with your registrar to map that domain to your BlogSpot account.