What is overhead? Are there multiple types of overhead, or only one? What are some examples?
The business meaning of overhead cost explains it best. From Wikipedia:
The term overhead is usually used to group expenses that are necessary to the continued functioning of the business, but cannot be immediately associated with the products/services being offered1 (e.g. do not directly generate profits).
Overhead is a "cost" you will incur to be able to perform an operation; you need to "invest" some resource to perform the operation in question.