It depends if you are doing proper Pilates, where they use the tables with the resistance bands, usually one to one or if it's a group class, based around floor exercises.
If it's a group studio class, the exercises are quite gentle. The idea with Pilates is that it is concentrating on core strength and lengthening muscles for leanness, so you will be doing lots of things which involves raising legs and arms and pulsing and holding specific positions.
The classes involve no Cardiovascular activity, purely toning and strengthening, although the idea is quite basic and appears easy, you will probably feel a little soreness, particulalrly in the ab region 1-2 days later.