I've got the CLS and clear window clip-filters. The clear window filter sits in the camera for normal use and is intended to keep dust out of the internals when changing lenses. The CLS filter is a light pollution filter for night photography.
They have a range of other filters, such as Ha, OIII, UHC, etc. which are offered with and without IR blocking for cameras that have and have not been modified (respectively). See
http://www.astronomik.com/en/clip-filter-system.html. Note that the filters that end with -CCD are for use with modified cameras.
As a tip, I've found that the tab on the filter's storage box makes an excellent tool for releasing the filter from the camera's body, rather than using a finger nail, which is what Astronomik show you to do.
BTW, returning to my previous warning about Canon EFS lenses not being compatible with clip-filters, the reason is that EFS lens' always protrude, by design, into the camera's body. In my limited experience of third party APS lenses for my Canon DSLR, they do not protrude into the camera's body, which is why I can use my clear-window and CLS clip filters with them.
Regards
Dave B.