What do contractions feel like?

I’m only 21 weeks with my first baby, I had some kind of bad cramping earlier but went away pretty fast - I don’t think it was contractions or anything like that, I wasn’t worried, but it had me thinking, I have no idea what contractions feel like.

Also, at my 20 week appt the nurse gave me some paperwork that mentioned I could get contractions anytime between now and when I go into labor. How common are contractions this early on?

Thanks!