Apparently people are always asking questions about the lions because there's information about them on the Art Institute's FAQ!:
The two bronze lions that flank the Michigan Avenue entrance were made for the Art Institute's opening at its current location in 1893. They were a gift from Mrs. Henry Field. They have unofficial "names," given to them by their sculptor, Edward Kemeys, that are more like designations. You'll notice that the lions are not identical and thus are named for their poses: The south lion "stands in an attitude of defiance," while the north lion is "on the prowl."
If you take a picture with the lions - a totally normal activity because they ARE wonderful - people will almost certainly think you're a tourist. This is possibly one of the most-photographed things in Chicago.