Most people would not admit to being prejudiced, but most people would admit that they allow their past experiences with various groups to influence their perceptions of others who fall into that same group; these two things, though, are one in the same. When an individual holds an opinion based solely on a person's classification with a certain group, the individual is prejudiced. This does not always have to be a negative opinion; it is a common prejudice, for example, that individuals of Asian descent are good at math and/or science. On the surface, that does not seem negative; when the individual is expected to do well in those areas, however, there can be unfair consequences or perceptions due to this existing prejudice. At its core, prejudice is little more than preconception; a person makes a judgment - subconsciously or consciously - about another based on some irrational or unfounded evidence.
Prejudice does not always have to apply to individuals, either. A person can be prejudiced against a number of things, including ideas or objects. While it may seem like a trivial example, a child who does not like peas may hold a prejudice that all green vegetables are unpleasant; they may refuse to try asparagus or broccoli due to their existing prejudice. The effects of prejudice can be profound, impacting the individual who holds the prejudice as much as the target. Individuals can even hold prejudices against themselves. Consider the individuals of Asian descent mentioned above; if a member of this group is not good at math, this prejudice they hold against themselves can have significant repercussions on their mental health, including increased risks of depression. While prejudice is a common part of the human experience, it is something that everyone needs to be aware of and work to eliminate.