Well there are all sorts of different ones and it's been quite a while now since I was looking but the one I met my partner on had a bunch of criteria you could look for. You'd enter the age range, gender, etc you were interested in along with that info for yourself, and stuff like whether you like or own cats or dogs, various activities you like and then there are a ton of random questions you can answer or skip. Answering the questions builds up a profile of sorts that it uses to match you with others who have answered similarly and it would show you a percentage of how well you ostensibly match to a given individual. When searching you could filter by basic characteristics like age range and gender, so if you said you were interested in women age 25-35 you'd never be shown a 50 year old man.
There are other services that cater to more specific niches.