Climbing is using one's hands, feet, or any other part of the body to ascend a steep object. It is done recreationally, competitively, in trades that rely on it, and in emergency rescue and military operations. It is done indoors and out, on natural and man-made structures.
Bouldering is climbing stripped down to its raw essentials. Leaving behind ropes and harnesses and just using climbing shoes and a bag of chalk over safety mats, your challenge is to climb short but tricky bouldering "problems" (a route, or sequence of moves) using balance, technique, strength, and your brain.
You don't need experience or lots of expensive kit to have a go - making it really easy to get into if you've never tried it before.
The climbs are high enough to be exciting, but not so high that they're hugely intimidating. Using safety mats means that the risks of falling off can be managed, and leaving the ropes behind means that you are free to concentrate on the climbing, not the equipment. It's just you, the wall, and your friends on the ground egging you on.
Bouldering is a great physical workout, using muscles all over your body - not just your arms. Core strength is a real focus, as is flexibility. But unlike some other forms of exercise, it's so much fun you might not notice that you've had a big workout - until you wake up the next day! If you're new to the sport, you'll find that you make big improvements in your climbing ability after only a few sessions, as you use and tone those rarely used muscles. If you want to see what regular climbing could do for your physique, just take a look at some pictures of famous rock climbers online - it's pretty impressive.
It also takes a bit of thinking, as often brute force alone won't be enough to get you to the top. You'll need to think about how to solve the various bouldering problems, try some different techniques and methods out, and hopefully eventually crack it. This makes a bouldering session totally absorbing, making your forget about your stresses and leaving your mind clear and refreshed at the end of it all.