Crab Grass

Now here's a familiar sight to everyone in the USA. The ever growing, ever invading crab grass. They are native species, but are generally considered a weed for it's tendency to spread into gardens and choke out plants that are meant to be there.
It's distinguishable in it's wide and flattened blades, it's thick stems, and it's rough texture. It's heavy and lays fairly flat to the ground when it's in a mowed lawn, they grow in a circle around the main root and have densely packed leaves.

They're insanely difficult to remove, but if you're not concerned about the absolute prettiness of your lawn they're not a problem.
It grows best in full sun and in warmer areas, especially next to concrete. There are several warm climate grasses that look similar to Crab grass like Buffalo grass or St Augustine grass, but this is the one that's sharing my yard with the Bermuda grass. I wish I could explain the differences, but grasses are difficult as they all look just about the same.