An Account Manager is a person in a marketing company who is directly in charge of all of the customer's accounts. But, he can have his own accounts that he services, too. He personally works with his clients to give them good customer service, technical support, and sales tips.
An Account Executive, on the other hand, is assigned to one or two particular clients and he comes under the supervision of an Account Manager. He, too, may have been the one to set up a particular account. Therefore, he knows his clients well and can interact with them on a one-on-one basis. He acts as a intermediary between his clients and the company he works for.