Being paranoid is when you are suspicious without reason. You may feel that people are trying to harm you in some way or that something horrible is going to happen to you.
Everyone has suspicious thoughts at one time or another but paranoia can range from milder forms, where the person retains insight and realizes their suspicions may be ungrounded, to extreme forms, where the person cannot distinguish reality from fantasy. It can make the person suffering from paranoia feel isolated because they may feel they cannot trust or depend on anyone. They may also feel angry, fearful, vengeful, guilt-ridden and can become very depressed as a result.