Microgreens are even smaller than baby greens. Other than that, though, they're just regular greens—plants such as kale, broccoli, and many others.
We're just started growing them, but only in winter. We got seeds specifically intended for microgreens. (We bought ours from Johnny's Selected Seeds, but other companies sell them, too.)
There's really no reason we couldn't grow them all year indoors, but in the summer we have so many other things growing. They're also a little pricier since each seed is only producing a fraction of what it would produce if it were planted in the garden and grown into a full-size plant. Still, though, in winter it certainly was nice to have flavor- and nutrition-packed organic greens growing in our basement!