How many types of validation controls are provided by ASP.NET ?

There are six main types of validation controls :-
RequiredFieldValidator
It checks whether the control have any value. It's used when you want the control should not be empty.
RangeValidator
It checks if the value in validated control is in that specific range. Example TxtCustomerCode should not be more than eight length.
CompareValidator
It checks that the value in controls should match some specific value. Example Textbox TxtPie should be equal to 3.14.
RegularExpressionValidator
When we want the control value should match with a specific regular expression.
CustomValidator
It is used to define UserDefined validation.
ValidationSummary
It displays summary of all current validation errors.
Note:- It's rare that some one will ask step by step all the validation controls. Rather they will ask for what type of validation which validator will be used. Example in one of the interviews i was asked how you display summary of all errors in the validation control...just uttered one word Validation summary.