People first, means you refer to the person first ALWAYS! No matter what diagnosis they have, they are not their diagnosis. This is true even when the disease or disability take over a large part of the persons life. Just as a person with cancer, is not cancer. Even though they may be spending a large portion of their time dealing with treatments, & the effects of it. This is true of any diagnosis. A person remains a person, & deserves to be treated the same as anyone else!
The most important label is their name. If that is not appropriate to use, any other term referring to them as a person is appropriate.
Many people don't want to be labeled, in any way, whether it's an illness, disability, race, faith, or style, they still want to just be themselves. Personal preference as to what they choose to be called, & what they call their problem matters! It also is sometimes made an issue, when it doesn't need to be. There are many times people over use labels, telling everyone they meet, or in every introduction when it's really not always every ones business. Avoid using one at all, except when you have to do so for a reason.
Disability - means a lack of ability, & is the respectful word to use when talking about someones disabilities. In fact it is the only general word for a disability that is politically correct, & has been for close to 20 years.
Retarded - is a out of date medical diagnosis. Mental Retardation (MR) was used to describe a large group of people who were different physically, or later intellectually. Those people were not treated as people. They were thrown together, with little regard for age, sex, problem, race, religion...All that mattered is they were away from the rest of the world. They weren't treated as people, or taught at all. Basically it really was a word that described people less than human. This is why it bothers many of us. To make things worse, it is now a word people use to describe something stupid, or that they don't like. That has made it Derogatory, & with it's history, even a slur.
Handicapped -Thought to come from a combination of two things. One word is having a cap in the hand. In other words the beggar. The second was from a game in England. The one w/the cap being the looser. A bit like old maid, it's not what you want to end up w/or be called. It is also used now to refer to inanimate objects. A word that describes inanimate objects, isn't a word to describe a person w/problems, or person in general.
Disabled -Disabled means permanently broken, or turned off. Since a person themselves can't be turned off, that doesn't work. The other almost sounds possible, but although a body can be broken, the person can still work on therapeutic treatments to regain, or help problems, making them never permanently broken. It also is an acceptable word to use regarding inanimate objects.
idiot - This is one most don't understand, but it was the word before retarded, used to describe the people that were put away from the public, & treated far worse than animals. There are people alive who still have this diagnosis in their past, so for me, & a few others, it's unusable.
Moron - Another word w/the same idea as Retarded, but is less thought about.
The first thing about respect of course is to refer to someone else, as they want to be refereed as. I call friends & family what they want to be called, it seems only right to do the same for others. Calling someone the name they wish to be called, as long as it isn't offensive to others is something many people do w/people they know, but the same rings true for what they call their diagnosis, no matter what it is. Around others with the same diagnosis they may not like the persons choice, so it's good to keep in mind that even commonly used words that describe a person, or group of people, can be offensive to many others.
It makes a difference to treat others as people, not their problems. It is a key to happiness & acceptance, & shows them respect for who they are.