Discrimination is the maltreatment of an individual or group of individuals based on their race, religion, gender or any other measure that is legally protected.
For example, an employer who fires or refuses to hire people of a certain color, simply because they are members of that race, would be practicing discrimination.
Essentially, discrimination is the unfair treatment or unfair judgment of a person just because they are a member of a certain group or have certain qualities.
There are many different types of discrimination, as well as laws that protect citizens from them. For example, it is illegal to discriminate against a disabled person in the work environment. It is also illegal to discriminate against people who are older (age discrimination).