if you check the
disABILITIES FAQs thread, there is one whole section about Guest Assistance Cards (GACs) in post #6 and there are some links to threads that should be helpful in post #3.
To answer your questions a bit more:
As the others mentioned, it's not a pass. It's a card that helps to tell the CMs what sort of assistance you need.
it says right on the card that it is not meant to shorten or eliminate waits in line, but to provide accomodation needed because of a disability.
Is it like a fast pass for every ride?
No. As mentioned in the GAC information in the disABILITIES FAQs thread, how it works at each ride kind of depends. And, even on the same attraction, it might be not be handled the exact same way the next time you come (even on the same day). How it is handled will depend on how busy it is, how many other people with special needs are there at the same time and the space available for waiting.
The CMs will attempt to take care of your child's needs. It just might not always be in the same way.
Is there a different line?
Sometimes there is a quieter space where you can wait. There is usually not a different line (people assume there are 'wheelchair lines' they can use, but most of the attractions/queues are wheelchair accessible, so there are not separate wheelchair lines.
Would we still need to get the fast pass for the rides we want?
Fastpasses work out well for many people because they give a fairly consistent experience - you get in the Fastpass line and in 10-15 minutes you are in the boarding area.
You can send one member of your party to get Fastpasses for everyone in your group. That person needs everyone's park pass to put in the Fastpass machine, but everyone does not need to be present.