I found this useful so I will update it.

properties.Cancel is now depreciated. Use Status instead. Options for status are:

CancelNoError
Continue
CancelWithError
CancelWithRedirectUrl(Obsolete - Default list forms are committed through asynchronous XmlHttpRequests, so redirect urls specified in this way aren't followed by default.)

Example:

            properties.ErrorMessage = "My error message";
            properties.Status  = SPEventReceiverStatus.CancelWithError;

For SharePoint 2013:

properties.Cancel is now depreciated, use Status instead. Options for status are:

• CancelNoError
• Continue
• CancelWithError
• CancelWithRedirectUrl (Obsolete - Default list forms are committed through asynchronous XmlHttpRequests, so redirect urls specified in this way aren't followed by default.)

Example:

properties.ErrorMessage = "My error message";
properties.Status  = SPEventReceiverStatus.CancelWithError;