ASP.Net Validation Control

Required Field Validator: is a control used when a user have to enter some data. If the required field validator stop the processing to the next page until the data have been enter in.

Range Validator is used when you have to check is everything is going right with certain option. For example how this validator works Well let say that we ar putting a number in the textbox you can set how high the number goes and how low the number can go into the textbox.

Reqular Expression Validator is used to set certain expression that you want to be enter in the textbox. If the expression doesn't match the expression that you have setthen you can used the system that the expression validator has.

Compare Validator is used when you have to compare two fields its more know to be used in Password or registration and the validation is also case sensitive. Is tell when the data have been enter incorrectly.

Cistom Validator is used when you want to used certain function.

Validation Summary Validator is used to display error message. Let's say that you want to validation message in one page for a better look. If you have set the properties of that validator in the area that say text set is to other validation control. This will ensure your validation works.

http://www.codersource.net/asp_net_validation_controls.aspx
http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.customvalidator(VS.71).aspx