There is almost always a single homepage which has links to other pages that are part of that "site". Sometimes it has links to pages that are part of other "sites" as well. (Note that net jargon uses the word site, which also means a physical place in the real world, to mean a web URL).
Websites can be used to advertise or sell things. They can also be used to talk to other people. A blog is a website where the location of the material is less relevant than who writes it, and which is more focused on dialogue. Very often the people who use blogs dislike the word "site" since it implies a controlled place. Sites are good for looking up information on the computer.