HTML (HyperText Markup Language) stands for Hypertext Markup Language. The goal of the language was to organize and manage content. In its most basic form, HTML enables you to create a page on the Internet. HTML code is what you write in order to post text, links, images etc., on your pages.
FLASH is a coded web language that enables the addition of animated content and video to a website. Flash files have the file extension .swf and need to be read by a Flash plugin in the navigator.
Web development is a broad term for the work involved in developing a web site for the World Wide Web. Web development refers to creating,building and maintaining websites. It includes aspects such as web design, web publishing, web programming, and database management.