There are many uses for this term, with subtle differences in meaning. The most technically correct usage is for a single HTML file that contains text and images, is part of a Web site, and has an individual file name assigned to it. When viewed by a Web browser, this file could actually be several screen dimensions long (appearing as more than "a page"). Many times, on the Web, a user must "scroll down the page" in order to view the rest of the contents on the screen. Even if "the page" prints out at ten pages long, that one HTML file is considered a single "Web page."
Large Web sites are said to have hundreds of pages of information. In this usage, "page" refers to the actual hundreds of separate documents varying in length, each with a different topic or subject. Web page length was once judged by how many lines of the content happened to fit on a printed 8 1/2" x 11" page, but this usage is out-of-date.
Digital information is not bound to the same restrictions as printed information. (There was once a time when graphic designers who had crossed over from the print industry and became Web designers would estimate a fee for designing an online project based on how many printed pages it came out to. This thinking is obsolete as Web designers realize the value is not in the amount of content but rather in how it is organized.) The term "Web page" also refers to an entire Web site.
You may hear someone ask, "Have you got a Web page?" This usage pertains to the collection of "pages" that are "housed" under one domain name. It is also referred to as a homepage.