A ClearCase Trigger is a an executable program that you create that you can make Clearcase invoke before and/or after a specified ClearCase event or operation. You tell ClearCase which commands/events to invoke it for, you tell it how to invoke it, and ClearCase will execute it at the specified time with the specified parameters, as well as set the environment with lots of ClearCase related environment variables to allow your trigger "script" to consult numerous items of contextual information at the time it is invoked.
Some folks have developed their own ClearCase triggers and/or wrappers and/or related APIs and have made them freely available for others to download and use (in an OpenSource? fashion). Here is a list of such sites where ClearCase-related code (usually Perl-code) may be downloaded:
CG Software Design is Christian Goetze's website. He is another ClearCase-related product and service provider with some open-source triggers freely available. (See the "ClearCase Extension Pack" at http://www.cg-soft.com/ )
ClearPerl is a set of Perl5 libraries by BradAppleton? & friends. It is no longer actively developed/maintained but the code is available for download at http://www.bradapp.net/clearperl
Sclc and CDiff are a set of Perl scripts maintained by BradAppleton? to count lines of code (for various languages and in various ways) and to produce customizable format context-difference listings on top of ClearCase's cleardiff program.