Types of spankings

A spanking (or flogging, belting, whipping, you name it) can be a punishment, it can be fun, and it can be an erotic experience.

There are several types of spankings, and they can be (loosely) classified as:
For fun
Sensual
Sexual
For no reason
Punishment
Breaking
Therapeutic
For relief

For fun

These are the spankings given playfully, with everybody around laughing and joking, like “birthday spankings”, or a couple of slaps on the butt as a joke. Their use is widespread even outside the BDSM community. It is the kind of spanking given when playing social “spanking games”, and also most of the ones given by vanilla couples as a sexual foreplay. They involve normally the hand, or at most a light instrument, and usually means little (at most some stinging) or no pain.
In this type, the pleasure is shared.

Sensual spankings

These are given for the sensorial experience in itself, to enjoy giving or receiving different sensations, to take the bottom to the ecstagony of pleasure. They are about caressing soft or violently, about touching the other, about feeling the other over your lap or feeling yourself on the other’s lap.
In this type, the pleasure is mainly for the bottom

Sexual spankings

They are part of the foreplay for lovemaking, or part of lovemaking itself. They are used for arousing the other, or to add to your own arousal.
The pleasure is shared
In both of last kinds, the instruments used and the pain level depends on the taste of the participants.

For no reason

Why ruining the pleasure of a good spanking with excuses for giving them? These are the spankings given because the dom wants to show who is in charge, or because the sub feels a need for it, because they both want to enjoy the game, or “just because…”. Of this kind are the “maintenance” spankings given by a “Master” to a “slave” just for reminding her of her condition. And also of this kind is the old advice of giving your significant other a licking once in a while (“Maybe you don’t know why, but she probably will”).
Of course, the instruments and pain level will depend on the whim of the players .
The pleasure is shared.

Punishment spankings:

These are real spankings, the ones used to produce a change in behavior. They are given for punishing some bottom’s real life or “in scene” mischief. As the ones given by the parents, these are supposed to be painful, not pleasurable. For them being useful, the punished should remember them as an experience to be avoided. The punishment spankings can involve some instrument and a position other than on the knees. Not touching the bottom other than with the instrument adds to the punishment. This is a good occasion for “six of the best” with a cane, or for a good paddling. The pain level will depend on the punished's sensibility. See Red butts.
This type is the only one in which the pleasure, if any, is mostly for the top.

Breaking spankings

These, even when they involve serious pain, are not as much physical as they are psychical. Its purpose is to break all of the bottom’s barriers, letting him/her cry her soul out. They are usually given in response to the bottom’s need to be reduced to a childlike state, where crying is permitted and nothing can be withheld.
Be careful if playing this way: you are touching deep into the bottom’s feelings, there can be a regression to childhood, and if the bottom was severely punished or abused as a child, it will bring back painful memories.
The aftercare should be especially intense. The bottom will probably will have to be caressed, hugged and kissed. S/he will have to be cared for, and to be reminded that s/he is a valuable human being, that is loved, respected and needed. Of course, a lot of pain is required for breaking the bottom’s mental structures.
Don’t try this with an stranger, or if your relation is not very stable, or if you are not totally sure of what you are doing.
No pleasure in these, but they are given at the bottom's request.

Therapeutic Spankings

These spankings (also known as "spanking therapy") are given for its curative value. All kinds of illness were "treated" with spankings, and there are still some uses proposed for them.
As in any therapeutic situation, they are not given for pleasure, but because they are needed.

For relief

During normal life, tension accumulates. A good spanking, with its crying and associated submission feelings, can relieve those tensions. Being punished is cleansing, letting the sub in a condition for beginning again with a clean slate. It could be used for relief of guilt, with a clear reason or for reasons not clear even for the spanked. Or it could be just to get relief for any other cause of stress produced by everyday life.
No pleasure, but good for the sub.