Medicare Part C is known as Medicare Advantage. These are private plans run through Medicare that, by law, must at least be "equivalent" to regular Part A and B coverage. But there's lots of variation among Part C plans. Any given one may cover less of one thing and more of another than Part A and Part B do. (Confusing, right?)
Some Part C plans provide significant coverage beyond what you get in Parts A and B - including, in some cases, prescription drug coverage - but not all. The better ones basically function like Medigap policies, but are administered by Medicare rather than being wholly run by private insurance companies.
In fact, if you have a Medicare Advantage plan, any Medigap policy you may have is useless; Medigap won't pay if you are covered by Medicare Part C.