A bureaucrat is pretty much a civil servant or a public servant, a person who works for the government. Bureaucrat jobs are also sometimes used to describe "desk jobs". Like Duane mentioned, there is a negative connotations associated with bureaucrat. A executive is someone that is in charge of managing a large organizations. Like a CEO (Chief Executive Officer), who is in charge of a company and makes all the major decisions.

I think the real difference is that calling someone a "bureaucrat" has negative connotations. It suggests the individuals are inefficient and unable to quickly react to changing circumstances. Bureaucracies are generally known for being very large, slow moving, costly, and inefficient organizations. The term "executive" carries a much more positive connotation and suggests that a person is important, powerful, and wealthy.