HTTP is a standard used for communication between Web browsers (such as Netscape or Internet Explorer) and Web servers (such as Apache or Internet Information Server).
HyperText Transfer Protocol
HTTP is the protocol which underpins the Web. You may recognise the initials from the beginning of URLs which, when given in full, read 'http://www.somewhere.com/'. The 'http:' instructs your Web browser to use the HTTP protocol when communicating with the server at www.somewhere.com.
A 'protocol' is a defined communication standard; a list of commands and responses which computers can use to communicate with each other.
A program wishing to obtain information from a Web server must connect to that machine and send an HTTP request to the machine. The request specifies the desired file and also includes 'headers' with additional information such as the name of the requesting program, cookies for the site, etc.
For example, a request for the file 'http://www.somewhere.com/something.html' would be made by connecting to www.somewhere.com and sending an HTTP request beginning 'GET /something.html HTTP/1.1'.
When the server receives an HTTP request, it replies by sending an HTTP response. This response contains some informational headers about the type of response (whether it is a successful response, an error, or somewhere in between) and giving the type (text, picture, etc.) of the file requested, as well as the actual data.