The customvalidator control allows writing custom validation for a control. So, if the others validators: RangeValidator, RequiredFieldValidator, RegularExpressionValidator, CompareValidator, is not useful for a specific validation, it is possible to write server side validation function or a client side validation function.
To use the server side validation function is necessary to program the ServerValidate event. To use the client side validation set the ClientValidationFunction.
CustomValidator Class MSDN reference.
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