Pilates is a form of exercise that is based on slow, measured, repetitive movements. It is intended to strengthen lesser-used muscles. It aids improved posture and abdominal control.
It's a bit like yoga but focuses on building your core strength and can be very good for posture and such.
It's a series of exercises which have different levels which you work up as you get stronger and more experienced.
The idea is that you pull in a number of internal muscles(pulling in through your tummy and up through your pelvis) and hold them during the exercises to get the benefit and a lot of emphasis is put on breathing in a certain way.