Event Receivers or Event handlers are created to handle the basic actions or events against an item,list\library, a web or a site. There are two kinds of events in sharepoint.
Synchronous Events: Like Itemadding (not added yet), Uploading (document not uploaded yet) etc.
Asynchronous Events: ItemAdded (after item is added), Uploaded(after a document is uploaded)
Events receivers can be written to override an event for e.g. ItemAdded event to change the name of an Item once it is added.