A clerk is an office worker who does various paper work and often keeps accounts of money or other details. An office clerk might answer the phone, file papers, or organize computer records.
A clerical worker is one kind of clerk, and another is the person behind a counter at a store who rings up sales on a cash register. The clerk at your favorite bookstore might also recommend the best new books. Clerk originally had a religious meaning, from cleric, which meant both "ordained person" and "literate person." Because of this second meaning, clerk came to mean "anyone who can read or write."