The method attribute indicates the HTTP command to use when sending request to server. The action attribute is the relative URL of page to which request will be sent. Two methods can be used with form data - GET and POST. When GET command is used the from data is appended to the action URL as a query string, e.g.
GET /getCustomer.aspx?Id=123&colour=blue HTTP/1.1
When GET command is used to send data then complete URL and query string can be seen and modified in the browser address bar. This allows users to easily bookmark or link to the results of the form - important for search pages, but not good for authentication pages where the users credentials will be visible in the URL. Not good where large amounts of data need to be transferred - Internet Explorer and IIS impose a limit of 1024 characters in URL.
The POST command is better for large amounts or sensitive data. When POSTing data is placed into message body:
POST /getCustomer.aspx HTTP/1.1
Sending data back to server as part of request often refereed to as postback. Although term derived from POST command, can also perform postback using GET command, ASP.NET webpage contains IsPostBack property that indicates if data is being sent back to server or if page is simply being requested.