Chicago tuning on a banjo is a tuning that is generally used on 4 string banjos and is the same four notes as the first four strings of a guitar or a baritone ukulele. This tuning is primarily used for plectrum banjos and has been for generations. Many tenor banjoists use this tuning as well, though it is better suited for the plectrum banjo due to the similar scale length a plectrum banjo and guitar have. In fact, I have seen five string banjo players use this tuning, though it is not as common.
As most of us know, standard tuning for a 5-string banjo is an open G tuning (G,D,G,B,D). Open G tuning means that if we strum all the strings without fretting any of them, we will be playing a G chord. Open G is a great tuning, but there are many other ways you can tune your banjo as well. By tuning it differently, you can get a completely different tone out of your banjo. It also can make it easier to play certain songs in different keys.