Websitecomputer of some kind, including sometimes ones in mobile telephones (see WAP).
There is almost always a single homepage which introduces the rest of the web - containing links to other pages that may be considered part of that "site" or or part of other "sites" (in net jargon which confuses a physical site in the real world with a web URL - which is not physical).
Websites can be used to advertise or sell things. They can also be used to talk to other people. A blog is a web 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.
A wiki is a web where the material being discussed matters more than who discusses it, or who is presenting it. The Simple English Wikipedia is a wiki.
Users can access any web (site, blog, or wiki) by using a URL. The homepage and the rest of the site usually has the same root URL - for instance, pages at the Simple English Wikipedia always start "http://simple.wikipedia.org/..." but then vary after that.
Web sites are usually presented in HTML (Hyper Text Markup Language) but are not always written or maintained that way - some use WAP and others use XML.