Everyone can be mistrustful at times, particularly if life hasn't treated him or her well. But people who are prone to paranoia always dread some forthcoming attack or betrayal. They are forever anticipating that something awful will happen, and trying to second-guess what their adversaries might do. They focus on their fears for the future, and take little account of the majority of times when the past has proved them wrong.
In milder forms, the person has some insight into what's going on and realises their suspicions might be groundless. In extreme forms, they can't distinguish reality from fantasy. (Psychiatrists may refer to this as a psychosis, and talk about people having delusions.) It can be a very isolating condition, because people feel they can't depend on anybody. They may feel angry, fearful, guilt-ridden, suspicious, vengeful and excluded, and may become very depressed as a result.