What are the CGI Variables and what do they do

SERVER_SOFTWARE
The name and version of the information server software answering the request (and running the gateway).
Format: name/version

SERVER_NAME
The server's hostname, DNS alias, or IP address as it would appear in self-referencing URLs.

GATEWAY_INTERFACE
The revision of the CGI specification to which this server complies.
Format: CGI/revision

SERVER_PROTOCOL
The name and revision of the information protcol this request came in with.
Format: protocol/revision

SERVER_PORT
The port number to which the request was sent.

REQUEST_METHOD
The method with which the request was made. For HTTP, this is "GET", "HEAD", "POST", etc.

PATH_INFO
The extra path information, as given by the client. In other words, scripts can be accessed by their virtual pathname, followed by extra information at the end of this path. The extra information is sent as PATH_INFO. This information should be decoded by the server if it comes from a URL before it is passed to the CGI script.

PATH_TRANSLATED
The server provides a translated version of PATH_INFO, which takes the path and does any virtual-to-physical mapping to it.

SCRIPT_NAME
A virtual path to the script being executed, used for self-referencing URLs.

QUERY_STRING
The information which follows the ? in the URL which referenced this script. This is the query information. It should not be decoded in any fashion. This variable should always be set when there is query information, regardless of command line decoding.

REMOTE_HOST
The hostname making the request. If the server does not have this information, it should set REMOTE_ADDR and leave this unset.

REMOTE_ADDR
The IP address of the remote host making the request.

AUTH_TYPE
If the server supports user authentication, and the script is protects, this is the protocol-specific authentication method used to validate the user.

REMOTE_USER
If the server supports user authentication, and the script is protected, this is the username they have authenticated as.

REMOTE_IDENT
If the HTTP server supports RFC 931 identification, then this variable will be set to the remote user name retrieved from the server. Usage of this variable should be limited to logging only.

CONTENT_TYPE
For queries which have attached information, such as HTTP POST and PUT, this is the content type of the data.

CONTENT_LENGTH
The length of the said content as given by the client.

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In addition to these, the header lines received from the client, if any, are placed into the environment with the prefix HTTP_ followed by the header name.

HTTP_ACCEPT
HTTP_ALLOW
HTTP_AUTHORIZATION
HTTP_CONTENT_ENCODING
HTTP_CONTENT_LENGTH
HTTP_DATE
HTTP_EXPIRES
HTTP_FROM
HTTP_IF_MODIFIED_SINCE
HTTP_LAST_MODIFIED
HTTP_LOCATION
HTTP_PRAGMA
HTTP_REFERER
HTTP_SERVER
HTTP_USER_AGENT
HTTP_WWW_AUTHENTICATE

To view what each of these header fields are, go to http://www.w3.org/Protocols/HTTP/1.0/spec.html#HeaderFields

Thank you to the owner of
http://hoohoo.ncsa.uiuc.edu/cgi/env.html
for providing the list of CGI Variables and their definitions.