This looks like the weed I have been trying to deal with in a garden at my daughter's school--I just thank goodness it's not growing in my own yard (though I do have every other weed possible I think). I hadn't been able to ID it until I saw it here, either this forum or Name That Plant. It's Sheep Sorrel (Rumex acetosella)I believe. Let me know if you figure out a way to get rid of it, please!

dbs has the ID right. It is quite a spreader because it has runners as well as seeds. You can pull all the plants and there will still be seeds in the ground that can grow. I've gotten good control using herbicides (curtail and roundup). In a lawn, neither of these products is appropriate. Where is yours growing?

I know this won't help you to get rid of it (as for how to instruct you on this I'm at a loss), but I thought I'd just point out that if you want to, you can actually use that weed- it's edible like yellow wood sorrel and can be added to salads and the like. SO.... ;)

Good luck on how to remove it!!