When the background blurs, it is called bokeh. Pronounced Boke-aay or boke-uh.
The definition is: "a photographic term referring to the appearance of out-of-focus areas in an image produced by a camera lens using a shallow depth of field. Different lens bokeh produces different aesthetic qualities in out-of-focus backgrounds, which are often used to reduce distractions and emphasize the primary subject."
I LOVE LOVE LOVE this feature because it can make a picture that has a less than perfect background look great. I love it when I take pictures of my kids and you can't see the messy house in the background because it is all blurred and you can't even tell what the blur actually is! The way I get my blur is with my favorite lens. I know a lot of people that have the 50 mm 1.8, and LOVE it too. As far as glass goes, the 1.8 is a STEAL! This glass creates amazing bokeh because it has the ability for such a wide aperture and creates a very shallow depth of field. A shallow depth of field means that your subject is in focus and the rest of the photo is not. I love using my lens wide open (meaning the lowest aperture number it will allow) without overexposing the photo. If your aperture is open too wide, that means too much time for light to come in. So I determine my aperture according to the light and go from there. But, I always stay as low as I can to create a nice bokeh! I love it. I love it. I love it. Another reason I love my 50 mm 1.4 is because it is so great in low light situations. We will get into that another time.
For a really great explanation on bokeh...go to... The Pioneer Woman. She explains things in a way that even I can understand. It is a FANTASTIC resource for all things photography!