BDSM really stands for BDDSSM because some of the acronym takes on double meaning. So we have:
Tying up or restraining your partner.
There are different ways to do this, and different materials you can use.
Restraints, usually of the wrist and ankles, can include:
An element of a Dom-Sub relationship. Generally defined as: training a submissive.
You'll want to distinguish discipline from punishment:
One partner exercising control over the other.
Usually the dominant partner is called:
One partner submitting to the control of the other.
I like to distinguish between power and control. Because really, the sub has all the power, they're just ceding control to the dominant.
And of course, if done properly, it must be consensual.
Usually the submissive partner is called:
There are other monikers too, but those are the main ones.
(Originating from the French writer Marquis de Sade)
Taking pleasure from someone else's pain.
You can take pleasure from:
Sometimes it's sexual, but not always.
Taking pleasure in your own pain; the other side of the coin from sadism.
You can take pleasure from:
The pain can be sexually stimulating.
So there you have it, those are the basics of BDSM.