In the most common words, a blogger is a person who blogs. A blog is basically a website that is written in an informal or conversational way. The blog contains posts, which are entries that can be on any topic, including personal opinions, activities, experiences, news, gossip, fashion, design, etc.
A blogger is any person who writes posts and shared them on a blog. The blogger may also be responsible for search engine optimization (SEO) on the blog posts, so that they are picked up by the search engines such as Google, Yahoo, etc. Many blogs are also often used as a personal journal or diary from the point of view of the blogger.
A blog can be run by a single person, or by a single group. Nowaday, many “multi-author blogs” (MABs) have also shown up. Here, the posts are written by large numbers of authors and professionally edited, similar to a print magazine or a newspaper. All of these people are bloggers.
Blogger is also the name of a blog-publishing service, run by Google. Blogger allows a person to set up their own blog and start writing content on it, hence effectively turning them into a blogger. Generally, the blogs are hosted by Google at a subdomain of blogspot.com.