Good questions.
As Judith said, the answer depends. This is a little long (and I hope not too confusing). Ask as many questions as you need to clarify things.
There is a general GAC card that has stamps added to it depending on the needs of the person with a disability. The general card has printed on it:
This card will allert our Cast Members to provide the assistance you have requested, where applicable. It is NOT intended to provide immediate access. (It actually says that on the back and front of the card). It also says:
At attractions offering Disney's FASTPASS, Guests should obtain a FASTPASS return ticket.
The stamps that they can add to the GAC include things like being allowed to use the wheelchair entrance; using a stroller as a wheelchair (so it can go in lines and buildings where strollers are not allowed); allowing an alternate waiting area out of the sun or in a quieter spot.
Many of the standby lines (and almost all of the fastpass lines) at WDW are wheelchair accessible. This is called Mainstream Access. Most of the rides/attractions at AK and at the Studio are Mainstream Access. At Epcot, many of them are or have been adapted to be Mainstream. At MK, the newer rides/attractions are Mainstream, but the older ones (specifically in Fantasyland) are not all accessible and in some of those, wheelchair users or other people with disabilities have a separate entrance, which is usually a shorter distance.
In some cases, boarding of guests with wheelchairs or
ecvs is done at the ride exit. The wait will usually be in the regular line and the bypass to go to the exit comes right before boarding. This allows for a little more time and space for boarding and also allows the wheelchair or
ecv to be at the exit when you get off. Someone with a wheelchair or ecv and no additional needs would not need a GAC because the CMs can easily see the wheelchair or ecv and route you to the correct place. Someone with an invisible disability that means they need to avoid stairs might have a GAC that says they can use the wheelchair entrances. So they might wait in the regular line until it gets to the wheelchair entrance and then follow that. Often, the line for wheelchair access is the same distance as the regular line (examples would be ITTBAB - which is a long walk thru the pathway around the Tree of Life, The Safari). Sometimes there is an extra wait for people with disabilities; because of fire code/evacuation, each ride/attraction can only have a certain number of special needs guest on/in it at a time. So, if you get there when they are at the limit, you will need to wait for someone to exit before you can go.
Are there specific rides/attractions you are interested in?