Cokin P Polarizer and other Filters

[Texas]

1) What is the difference between a linear and a circular polariser filter and how/when should they be used?
and

2) Having achieved focus, will it not cause the focus to go off if the filter is then 'played with' after acheiving focus?
plus

3) Why would I need to use colour filters when my camera is digital and images can if necessary be filtered by photoshop elements.

Any explanation would be really useful and apologies if this is a bit basic.

Thanks
John