It's lurverly...although probably a bit nervous giggly for the complete beginner, as you try to work out how to put all your limbs in the suggested position without falling over/farting/your t-shirt sliding over your head.
But very, very good for stress and when you get accomplished (and motivated) you can do it at home.
As is customary on Mumsnet, someone with more experience will be along shortly, but for now, know that yoga comes in different flavours, all of which run through the same set of stretching and strengthening postures, with the aim of clearing your mind ready for meditation (or in my case, sleep ) at the end of the class. This is actually the main point of yoga, which is basically a spiritual discipline, but I've not come across any classes yet where people force you to sign up to chanting/meditation/anything spiritual if you don't want to.
Some forms of yoga (Iyengar, for example) put more emphasis on doing a few postures very correctly and are good for beginners as a result. Others (like Ashtanga, which is the form I'm most familiar with) are more dynamic, which is to say more gymnastic effort is required to move from one posture to the next, which can be quite exhausting, although very good for clearing the mind.
British Wheel of Yoga is the national governing body and I think their website might be a good place to start for finding a good local teacher. All you'll need to try a class is a towel (maybe not even that if you don't mind lying directly on a floor) some fairly well fitting but stretchy clothes and bare feet.
Enjoy!