This is Galium aparine, which has many common names including goose grass, cleavers and robin-run-the-hedge. It's an annual weed, native to Europe and Asia that is now naturalized throughout the world.
Identifying characteristics are the whorled leafs around the ribbed stems, and the hairs that cover the leaves, stems and fruits. Those hairs are hooked, and can attach themselves to animal fur or clothing, which helps with spreading the seeds.
Control can be done by pulling the plants before they go to seed. The stems break easily (because it's part of their survival strategy), but if you break them at ground level, I haven't noticed them regrowing. They will also respond to herbicides if you're willing to use them.