One of the most common identification challenges that I run into when working at Wild Birds Unlimited is trying to separate Black-capped and Carolina Chickadees. Many people call all chickadees Black-capped by default even though that is not the species that occurs in many areas around the country. Part of the confusion with identifying these two species is there is a large area where their ranges come together, and they will actually hybridize, making telling the species apart almost impossible. Range maps for both types of chickadees are below.
In addition to having different ranges, there are small variations in the plumage of these species. As you can see in the photos below, the Black-capped Chickadee has a white nape (the back of the neck) and white edges on its secondaries, where the Carolina Chickadee shows a much more gray nape and more gray in the secondaries.
Let us know what species of chickadee you are seeing!