When you surf the web, web pages (made in HTML, DHTML, XML etc) and the "media objects" that are linked to them (images, audio, video, Flash, AVIs etc) are transferred from servers (where web sites are stored) to your computer browser software (Safari, Internet Explorer etc) via a language called HTTP.
HTTP is like a "postal service" for web pages. It carries the pages from the server to your computer.
FTP (File Transfer Protocol)
FTP is simular to HTTP but is most often used to upload web site pages (and linked media) from the computer they have been created on to the server where the web site will be stored.
FTP is just another "postal service" for transferring files across the internet. If HTTP is the "national postal service" then FTP is "Federal Express" or "DHL"!
Dreamweaver has tools to help you build a local web site and then using FTP upload it to your web server.
Other transfer protocols
However, whilst a browsers and HTTP are the usual method for retrieving and viewing files from a web server, FTP is not the only way of "uploading" them.
HTTP upload, Webdav, frontpage, FTP and disk sharing are just some of the other ways of getting pages onto sites. Despite being the most common, FTP is the oldest and least secure way of uploading. FTP over Secure Sockets solves some of the problems with the original FTP. FTP is horrible because it exposes unencrypted passwords on the wire, which means anyone snooping the wire can watch what you do, and break into your site.