An "error provider" is simply a DotNet object that can be associated with an 
individual control.  When you set its error message property, it suddenly 
become visible as a little red circle next to the control (you can set the 
icon to be whatever you choose), and it has a tool tip that displays the 
error message.

--MikeStallings (on behalf of FormValidation)

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BTW, these are typically called 'Server Validators' (or just 'Validators'), at least as of .NET 1.0 RTM.  (I didn't participate in the original beta - maybe they were "error providers" back then?)  At least, they're called so in the context of ASP.NET - I think validation exists in Windows Forms, but I haven't worked with it.

Server Validators validate the content of that object (the validator is itself a .NET server control with a C''''''ontrolToValidate property, which you set to the control-you-wish-to-validate's ID property).  Examples include a R''''''equiredFieldValidator (value of control can't be empty), C''''''ompareValidator (compares the value to another control, e.g. comparing the 'password' and 'repeat password' fields), and R''''''egularExpressionValidator (naturally, validates the content of a control via regular expression).  There are several more, and you can inherit from B''''''aseValidator to code your own.  Typically these are used to validate T''''''extBox controls, but any control marked with the V''''''alidationPropertyAttribute (which specifies the public property of that control that should be validated) can be used in the C''''''ontrolToValidate property of a validator.

Reference:  http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpref/html/frlrfsystemwebuiwebcontrolsbasevalidatorclasstopic.asp

--JosephRiesen