Ableism is the oppression of people who have or are perceived to have any form of disability, including physical, psychiatric, developmental, and/or intellectual disabilities. You may not think you're as familiar with this type of discrimination as with racism or homophobia, but think again. At school, have you ever heard people call each other "retarded" as a joke or an insult? This is probably one of the most commonly visible forms of ableism.
The disabilities that we can often see, on the surface, are physical disabilities-perhaps somebody has to use a wheelchair or has a prosthetic limb. One way people are ableist against physically disabled people is by not employing them for jobs within their capabilities. Somebody's disability shouldn't prevent them from getting a job if the job doesn't involve activities that are impossible or dangerous for them. And yet, the unemployment rate for people with disabilities is 8.9% compared to able-bodied people's unemployment rate of 4.3%. Another form of ableism against people with physical disabilities is not providing the same access to certain things that able-bodied people have. If businesses have stairs but lack a wheelchair ramp, making the building inaccessible for people in wheelchairs, they are, possibly unwittingly, being ableists. People are also doing this when they park in handicapped spots, except now they know that they're making a disabled person's life harder.
In an entirely different way, people can be ableist against people with neurological disabilities. This type of ableism probably surfaces most in schools, where people use the word "retarded" in offensive ways. Ableism against people with these disabilities is a complicated thing, because others might not realize that they have them. People might treat people with neurological disabilities differently because they think they're just not as smart or social as them.
You can help fight ableism by being inclusive of people with disabilities whenever you can. Don't judge people immediately because they're in a wheelchair. Be aware of neurological disabilities, and don't be mean to people because they might have one. Spreading awareness is one of the best ways to fight ableism.