Looks like a flat leaf parsley to me

What do the leaves smell like?

Chueh,
The seed may have been a gift from a bird, mouse, squirrel, or may have been in your mulch if you purchased it. In any case, you now know a good spot where cilantro will grow. I'd use what leaves I could, let the plant set seed since it's already flowering, save the seeds and sow them nearby so it will have the same conditions. It's a cool season plant and will bolt(as it has)fairly early. I sow it a few times during the season, freeze the largest leaves for off-season use and let the seeds fall where they may.

Yep, looks like cilantro that is bolting. YOu'll get a nice harvest of coriander seeds if you leave it alone.

Yes, its in several different growth stages now. The taller ones are the bolting leaves. All can be harvested, but cut away the stems that are at the thinner fern type leaves as they are a little tougher.