HTML is used to create web documents and consists of text and markup tags. There are two types of tags - container tags that define sections of text, and empty tags that represent single instructions. HTML documents have a HEAD section for metadata and a BODY section for visible content. Common file formats used in HTML documents include GIF, JPG, and PNG graphics, as well as multimedia formats like AVI, MOV, MP3, and WAV. Plug-ins allow browsers to display interactive content through technologies like Flash, Shockwave, and QuickTime. XML is used for custom data presentation and VRML creates 3D virtual worlds.