Hey, Eilat. So, a hematologist is someone specializing in blood disorders. I think of hemophiliacs (people whose blood doesn't clot properly) or maybe people who's blood clots too quickly.
A hematologist/oncologist is a person who specializes in both blood disorders and other cancers. This is most commonly the route people take because it gives them extra expertise in blood cancers like leukemia. This is maybe 1 or 2 years of extra training in addition to the hematologist's training. I think some programs make you do both. Most oncologists are actually hema/onc doctors. Finally, a pediatric hematologist/oncologist is simply a hema/onc doc who specializes in treating children. These doctors are actually trained in pediatrics first, then do a fellowship for hema/onc after their three years of pedatrics residency.