What kind of spider....?

Ok, there are a few spiders I've been seeing around the house lately. Trying to figure out what kind of spider they are, to know if I should be wary of them or if they are harmless. The spider almost looks like the pictures of a common house spider but the abdomen has a thin black line down the center and for the most part, they look almost all grey. I've been looking at spider pics for the past like three hours and all I'm doing is creeping myself out, lol. Anybody know insects pretty well? Would appreciate any help. Thanks!