Ableism is not simply prejudice, but rather the combination of prejudice and institutional power. Anyone, regardless of physical or non-physical abilities, can have ableist prejudices, biases, or tendencies. However, in the United States, being nondisabled means having institutional power and privilege; therefore, ableism here is the systemized discrimination of disabled people due to the societal belief that “normal ability” is superior because it is the majority. Ableism in America is systemic because it plays a role in our institutions and society, whether we recognize it or not. To truly understand ableism is to also understand how it is embedded in institutional and cultural systems, rather than focusing on an individual's thinking or actions.
Disabled individuals may sometimes be a party to systemic ableism, without intending to act in that manner, and simply because ableism is so ingrained in our institutions, cultures, and societies. For example, a wheelchair-enabled individual may harass someone with an invisible but chronic disability for parking in a disabled parking spot.
Microaggressions can target any group, but certain microaggressions specifically people with disabilities. Today's casual language includes a number of sayings that most people use in a way that they don't intend to be hurtful, but are certainly microaggressions and can be considered offensive by some people with disabilities, particularly because it diminishes the extent to which people have to live with their disabilities. Some examples include asking people if they're deaf or blind when they don't hear or see something, or stating that you're OCD simply because you like things neat and orderly.