A blogger is a person who keeps a blog as a major activity in their life. This is a diary or journal on the web (aka 'weblog'). Blogs are usually free to read, but nevertheless some have made fame and fortune for their writers. Many politicians and journalists are bloggers, perhaps realising that this is one good way to contact the younger voters.
Twitter and Facebook are networking sites which offer some of the functions of a blog. What a blogger can do on sites like that is limited by the proprietary software. The really professional bloggers have their own sites, and also hold pages on social networking sites.
