The colour of the holds usually indicate the route you can take. Sounds dangerous, but it's not - to stop you from falling and injuring yourself you are tied to a rope.
Bouldering is rock climbing without the ropes. The walls are lower off the ground and there are crash mats underneath in case you fall.
It's a sociable sport and welcomes climbers of all levels. With hundreds of climbing walls around the UK - you don't need to live near mountains to get involved.