CGI programs are widely used in processing forms. You can find information about other usages of CGI on Form and other examples and links page. However, I will try to explain the usage of CGI in forms briefly. A form is a special type of an arbitrary web page, as it allows user to enter the data required. Then, this data is processed by CGI program and converted into meaningful characters. The complete forms specification can be found here. 

I will not go into the detailes of form fields. Interested reader should go to my links page. 
There are two methods to submit forms: 

GET Method: 
When the submit button is pressed, the contents of the form will be assembled into a query URL like this: 

action?name=value&name=value&name=value 

Strange characters in any of the "name" or "value" instances will be escaped as usual. 

For empty fields, "name=" part of the query string will still be present. 

For checkboxes and radio buttons, the value attribute specifies the value of a checkbox or radio button when it is checked. Multiple radio buttons intended to have "only one of many" behavior should have the same "name" and different "value"s. 

POST Method: 
The contents of the form are encoded exactly as with the GET method. But, instead of appending the contents to the URL specified by the form's ACTION attribute as a query, the contents are sent in a data block as part of the POST operation. The ACTION attribute (if any) is the URL to which the data block to be POSTed.