That's an excellent question. A kinase, in most general terms, phosphorylates something. What that something is doesn't really matter. That's just what a kinase does. More specifically, kinases catalyze the transfer of a phosphate from something of high transfer potential to something of lower transfer potential (therefore it is an exothermic process). So an example could be transferring a phosphate group from ATP to a sugar, which is what PFK-1 does. Another example could be transferring a phosphate from a high-energy substrate like phosphoenolpyruvate to ADP, as pyruvate kinase does. So in short, their substrates could be either organic molecules or ADP.
And to add another complicating twist to the idea, a kinase might even be named for the reverse reaction it runs! For instance, in glycolysis, the enzyme that transfers a phosphate group from 1,3-bisphosphoglycerate to ADP is called phosphoglycerate kinase. At first glance, you'd think to yourself based on what I said above, "Hey, wait a minute. I thought kinases phosphorylate things." Well, yes they do! So that specific reaction is reversible (and remember enzymes catalyze reactions equally in both directions) and the kinase is named for the reverse reaction it runs, i.e. putting a phosphate group on 3-phosphoglycerate! So here, in one direction, it phosphorylates ADP from 1,3-BPG - glycolysis. In the reverse direction, it phosphorylates 3-phosphoglycerate using ATP - gluconeogenesis. Depending on which pathway is active, the same enzyme catalyzes both the forward and reverse reactions!
