Diffraction depends on the wavelength of light, but any lens will diffract light.
A lens that is limited only by diffraction projects your image onto the sensor with some softness related to diffraction. For instance, an infinitesimally small dot will resolve as (roughly) a small dot of diameter equal to about the f-stop in microns. At f/2.8 you're getting a maybe 2 to 4 micron dot, depending on what color of light we're talking about. It's not a dot, it actually smears out all across the frame, but it's "mostly" a dot, so we think of it as a dot.
Generally people assume that as long as this theoretical dot is smaller than the dots their sensor sees, then diffraction "doesn't count" which isn't quite right, but is pretty close. So, yeah, your normal crop sensor from about now runs up against diffraction limits around f/5.6 or f/8.0 or somewhere near there, depending on colors and the details of the sensor and so on. A full-frame sensor with the same number of pixels won't hit diffraction limits until later, since it's using big fat pixels. A D800 which uses a huge number of itty bitty pixels again, will run into limits about the same place a current crop sensor does.
Basically, don't worry about it too much. When you stop down a whole bunch, you're gonna get a little softness.
Search terms: Rayleigh Limit, Airy Disc
