Image Stabiliser.
Other names are OIS, OS, just to name a few
They are helpful in low light condition or telephoto zoom distance where you don't have a tripod and cannot afford shutter speed fast enough for your hand to be steady to take the shot.
So far, I only know pentax and sony alpha series have inbuilt IS in their camera body. C and N need to buy lens with IS on lens
There are alot of stuff you need to learn if you are really interested in photography. I have friends who are interested in photography and brought a DSLR only to find out that they just want better performing camera for nicer facebook photos. They went for C being brand concious only to find that they still can't shoot proper low light portrait. Please think about what you really want, when you posted the thread to sell your K20 I was like O.O already...
Pricing different between canon lens with IS and no IS:
EF 70-200mm f4L IS USM - $2299 (RRP)|$1862 cash (SLRR, Jun10)|$1883 cash (CP, w/ GST, Jun10)$1873 cash (AP, w/ GST, Jun10)$1890 cash (JO, Jun10) $1870 TK w/ GST Aug10
EF 70-200mm f4L USM - $1399 (RRP) |$1145 (MS, w/ GST, May10)$1120 (MS, w/ GST, Jun10) $1020 cash CP-P Aug10