depends but its a lot of very precise exercises based around good posture, getting better muscle control (ie focussing on exercising particular muscle groups) . you can use equipment to help (eg something called a reformer that is like an exercise bench with pulleys you can use to add extra tension on various exercises).
In terms of what its most like -its the stretching or precise free weights you might do in a gym or certain yoga or body balacne exercises.
does it hurt? well you do feel you've done somehting but I've never felt as stiff or painful as after a very vigorous yoga session or gym session.
it also makes a huge difference to posture and muscle tone.