My hair is really thick and curly, and I have it done every time I get my hair cut. For me, it has worked really well. They use thinning scissors, which have a blade that is wider and more square than blades on normal scissors, with a wide tooth comb in replacement of the other blade. They take small sections of your hair, and go through it with the scissors. They cut the hair so that some strands are short, and others are long. When it is done right ( thinning out the hair underneath, rather than the top) it isn't noticeable. Just make sure thin out the underneath layers of your hair, because if they don't it will look uneven.
Usually...they wash it...condition...put detangler...set it (put the rolls in) and then blow dry it. It works well. The effect is temporary though. It last a couple of days.
IF you are like Spanish or white or you have like thick but non nappy hair, you can get a Brazilian straightening ---which is when they put an intense amount of heat in your hair and it keeps it straight for months.
Sometimes they use a special pair of scissors that has chunks missing so that it doestnt cut all of it or sometimes they use a razor it depends on the stylist. It works just make sure they don't thin out the top layer of your hair if your wearing it down cause it will frizz out or will look choppy