Difference between Braxton Hicks and real contractions ?

I have read of some going into preterm. I’ve been having Braxton Hicks today but what’s the difference between them and real contractions? Like how will I know real ones? I got worried earlier because my stomach tightened and it just felt like a real sharp pain going across like a knife slowly going. I know real contractions get closer together and I don’t feel like getting closer like I have maybe 2 -3 pains an hour? But they hurt! There all about the same amount of pain. But my stomach feels so hard. Is that normal? I have felt very light movement today my baby is usually very active but today I’ve only felt him move once. And are contractions in a certain spot or anywhere? I’ve been having them everywhere lower and middle stomach but past few ones have been just been middle stomach. I’m not bleeding. But I am having alot of discharge, white mucus? Slim I guess? I’ve had to change my underwear this morning. My dr has not returned my call and I’m wondering if I should head to an Er or just relax on the couch.

And I’m 24 weeks.