Displacement -- One way to avoid the risk associated with feeling unpleasant emotions is to displace them, or put them somewhere other than where they belong. A common example is being angry at your boss. Displaying that anger could cost you your job. You might be afraid that you can not contain it, but also afraid of what will happen if you express it toward your boss. You might instead express it, but redirect it toward some other, safer source, such as your partner or best friend. You yell at them and pick a fight. They will forgive you or ignore it, and then you are able to express your anger but without risking your job.
Sublimation -- Related to displacement is sublimation, or the healthy redirection of an emotion. Instead of punching your boss when angry with him, instead of taking out your anger on your friends, you go to the gym and punch a punching bag. Other examples include turning the painful loss of a child into a campaign to increase child safety laws, turning a generally high degree of aggression into professional football, and turning the pain and resentment of a physical injury into a drive to overcome a disability.
Projection -- Projection is something we all do. It is the act of taking something of ourselves and placing it outside of us, onto others; sometimes we project positive and sometimes negative aspects of ourselves. Sometimes we project things we don't want to acknowledge about ourselves, and so we turn it around and put it on others (i.e., "It's not that I made a stupid mistake, it's that you are critical of everything I do!"). Sometimes it is simply our experiences (i.e., "My father was a reasonable man when we disagreed, so if I use reason with my boss we can work out our disagreement").
The problem with projecting negative aspects of ourselves is that we still suffer under them. In the above example, instead of feeling inadequate (our true feeling) we suffer with the feeling that everyone is critical of us. While we escape feelings of inadequacy and vulnerability, we nonetheless still suffer and feel uneasy. The more energy you put into avoiding the realization that you have weaknesses, the more difficult it eventually is to face them. This is the main defense mechanism of paranoid and anti-social personalities.
Rationalization -- Rationalization is often called the "sour grapes defense." This comes from one of Aesop's fables. The fox wanted some grapes, but could not reach them. This caused him to feel pain, as he could not have what he wanted. He rationalized, "They were probably sour anyway" to turn them into something he didn't really want, and thus couldn't really be upset about not getting. It is an intellectual way to diminish pain or guilt. The old "They're 50% less fat so I can eat twice as many" routine is the same. You make up a "logical" argument to avoid guilt.
Fantasy -- Fantasy can be a good or a pathological defense. Fantasizing involves creating an inner world when the real world becomes too painful, difficult, or stressful. Thinking about your upcoming vacation when work gets stressful is a healthy use of fantasy. However, if you don't solve problems, but only daydream about them being solved for you, if you avoid potentially problematic responsibilities and only fantasize about having rewarding challenges and experiences, fantasy becomes too much.
Intellectualization -- Related to rationalization, intellectualization involves removing the emotion from emotional experiences, and discussing painful events in detached, uncaring, sterile ways. Someone who intellectualizes becomes very distant from their feelings, and when asked to describe their feelings may find it difficult. They may understand all the words that describe feelings, but have no idea what they really feel.
Denial -- Denial is the simplest defense to understand. It is simply the refusal to acknowledge what has, is, or will happen. "My partner didn't have an affair, but was simply traveling for work a lot." A related defense is Minimizing. When you minimize you technically accept what happened, but only in a "watered down" form. "Sure, I have been drinking a bit too much lately, but it's only due to stresses at work; I don't really have a drinking problem since this is situational and not an inner weakness or something."
Repression/Suppression -- Repression is often thought of as the parent of all defenses. Repression involves putting painful thoughts and memories out of our minds and forgetting them. All defenses do this to some extent. Traditionally, repression is unconsciously "forgetting," that is, forgetting and not even realizing that you are doing it. You have no conscious memory or knowledge of that which is repressed. Suppression is when you consciously forget something, or make the choice to avoid thinking about it.
The problem with repression is that the memory, feeling, or insight repressed doesn't go away. It continues to effect us because our unconscious gives it a life of its own. It becomes all the more powerful because we repress it, and it can effect our decisions, reactions, etc... in ways that we don't see but others may.
Withdrawal -- Withdrawal is a more severe form of defense. It entails removing yourself from events, stimuli, interactions, etc... that could remind you of painful thoughts and feelings. Withdrawal takes several forms, such as silence, running away, and drinking and drug use. Talking to friends could prompt them to ask about painful events, so you avoid them. Television, books, coworkers, etc... can all remind you of unpleasant feelings, so you avoid them. Paired with fantasy, it can be paralyzing. Withdrawal inevitably leads to strong feelings of loneliness and alienation, however, which generally means you feel more pain.
Reaction-Formation -- This is one of the most difficult defenses for some people to understand. When we have a reaction that is too painful or threatening to feel (such as intense hate for someone with power over us), we turn it into the opposite (intense liking for that person). That way, we aren't threatened by the feeling, or even the awareness of the feeling. Like denial and repression, you can begin to do this automatically and as a result never know what your true feelings are.
Summary -- Defenses are ultimately something we do to protect ourselves from pain. While we all use them when troubled, we generally come to a point when we face our problems and don't have to rely so heavily on our defenses to protect us. Defenses become unhealthy when we refuse to face our true experiences, thoughts, and feelings.