It's a cardamine, probably Cardamine hirsuta, aka shotweed or bittercress. It is an annual weed, appearing when temps are cool (spring and fall) but can appear year round if conditions are right. You need to keep it from flowering and setting seed. I find it easy to remove from any planting beds but it germinates rapidly and grows fast so you need to keep on top of it. Mulching well helps. If widespread in a lawn, you can use any lawn weed killer and mow frequently to keep it from flowering until you have the infestation under control.

Thank you. There's a lot!

I am in zone 5/6 and bittercrest shows up in the fall, lays quite as an evergreen groundcover all winter and blooms in early March. If you use something like Preen in the late summer you can limit germination. I believe it to be an annual. Regardless it is a monster. Seeds spread up to 20 feet and can be in the soil for up to 40 years. Do your best to not let it flower. I have been able to contain it if I put Preen down at just the right time. If you have the energy in the fall, pull it out as it shows because in my zone it blooms before the weather is warm enough to want to be outside. Good luck. I've been fighting it for years.