The 4017 takes a clock pulse in and then steps the output from negative to positive in a series of ten steps, with only one pin being on at a time. It has the unique capability of counting up to a certain number and then restarting the count, counting up to a certain number and halting, or it can be cascaded to more 4017's for a higher count.
The outputs are labeled 0 through 9. It can sink about 10 ma. of current per pin and is a very versitile IC. It operates from 3 VDC. to 15 VDC.