What it is:
An account executive is a person responsible for managing a relationship with a customer.
How it works/Example:
For example, let's say John Doe works for an advertising agency. The agency would like to have Company XYZ as a client. As the account executive, John's job is to meet with the executives of Company XYZ, understand what they want, tell them what his agency can provide, and then get them to sign on as a client. When Company XYZ becomes a client, John's job is to ensure that Company XYZ is happy with the creative work his agency is producing and that the accountstay with John's agency.