HTTP is the protocol for transfering Web pages. Current version is 1.1, which is now an RFC on the standards track. It replaces the widely implemented 1.0.
Note: this is not related to Apache version numbers!
Uniform Resource Identifiers or Names (URI, URN) are the generic names for Uniform Resource Locators (URLs), used to identify resources on the WWW and Internet.
Cookies let you maintain state with the client, or track 'clickstreams'.
Hypertext Markup Language is the protocol used to design Web hypertext pages. Current widely used version is 2.0, often with extensions. Version 3.2 summarises the current practise.
CGI is the common gateway interface, which specifies how web servers can call external applications (scripts, programs or other gateways).
CGI provides a simple way of running programs on the server when a request is received. However they can be inefficient because they need to be started each time a request is made. There are various ways of creating more efficient dynamic responses.
Server-Side Includes are a way of writing commands into normal HTML files. When the HTML file is served to the user, the SSI commands are parsed and executed. Apache implements standard SSI, or you can use an alternate module for more advanced SSI implementations
Imagemaps come in several flavours: old-style NCSA cgi-bin program, new Apache imagemap module and client-side imagemaps.