An Event Handler is a .NET assembly that uses the Event System to hook into events occurring in the Content Manager. An Event Handler is triggered when an event occurs, for example when a Component being saved. The Event System is part of the TOM.NET (.NET Tridion Object Model) API.
Note: If you upgraded to SDL Tridion 2011, SDL Tridion 2011 SP1 or SDL Tridion 2013 SP1 from an earlier version, your existing event code, based on the TOM API, still works when called from the GUI or through the deprecated TOM API. But note also that if you create new, TOM.NET-based event code, such code is also called from the GUI. This means that a GUI event could trigger both your old and your new event code. For this reason, it is best not to mix the two.