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 (also known as 'weblog'). Blogs are usually free to read, but nevertheless some have made fame and fortune for their writing. Tavi Gevinson's blog is an example. Many politicians and journalists are bloggers, perhaps realising that this is one good way to contact the younger voters. Michelle Malkin's blog is an example of this kind of use.
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.