In library terms, both of these formats are used to communicate information in articles. Some databases serve up articles in an HTML format -- like a webpage -- and some serve up articles in a PDF format, usually as a separate download. Either way, the content (text and pictures) of the articles is identical, but the formats and layouts may differ slightly.
PDF files require a special PDF viewer on the web browser or Adobe Acrobat Reader, whereas the html files are computer formatted and do not require additional software to read them.
