Nipple clamps for beginners
Nipple clamps are sex toys used in nipple play, to stimulate the erect nipple based on the restriction of blood flow to that area.
The basic principle is to pinch the nipple, but unlike when you pinch using the fingers or gently with the teeth, the pressure can be maintained if a nipple clamp is used.
Whilst most people think of nipple clamps as rather kinky, some very soft nipple clamps can be purchased that apply only a very mild pressure to the nipples so they are both great for beginners or those who hate even moderate pain, and do not find it at all pleasurable.
Several types of nipple clamps can be purchased:
Tweezer clamps: Benefits are that pressure can be adjusted and pressure is milder and more gentle than other types. Often used by beginners to nipple play
Clover clamps: Based on a spring mechanism, clover clamps often come with a chain in between or with strings on the individual clamp so that if pulled on, the tension is increased. Most often purchased by those more experienced in nipple torture or for BDSM play
Rubber tipped clamps: These very often, but not always, have a screw so that you can adjust the tension of the nipple clams
Clothes pegs: Some people use clothes pegs as nipple clamps, and also special, small chrome clothes pegs can be used
Vibrating nipple clamps: It is possible to purchase nipple clamps which vibrate, further stimulating the erect nipple and the vibrations can be gradually increased as desired
Y-style clamps: As the name implies, y-style nipple clamps form the shape of a Y, with the 2 clamps attaching to the nipples and the third for genitals – either clitoris clamp or cock ring
If any numbness is felt during the use of nipple clamps, remove them right away.

Further reading
Nipple clamps on Wikipedia
Clips and clamps – a guide to all kind of kinky fun clamp toys
