iTunes has main categories and subcategories to choose from for your podcast. These categories are included in your show's RSS feed and help listeners find new shows through search and in recommended lists (Apple also populates top podcast lists by category in addition to overall). 

To designate your podcast's categories, you can do so when creating a new podcast or editing an existing. The field in Megaphone is called Podcast Category and pulls from the categories and subcategories accepted by Apple.

To edit categories on an existing podcast, select the podcast from your Podcast's library and click "Edit Podcast" in the upper right corner. Scroll down to the "Podcast Category" section where you can add or remove new categories, then click Save.

Visit this link to see a full list of all categories and subcategories.