The term Law Enforcement Agency (LEA) refers to the government agencies responsible for ensuring laws are enforced. While some of them are called police, others are called sheriffs, constables and Marshals. Outside of the US, such organizations are part of police services.
Sheriffs are County Police Officers who provide basic police and investigatory services for unincorporated areas, or cities that contract for services who don’t have their own police forces.
U.S. Marshals are usually judicial positions. Marshals provide security to Federal Courts, apprehend fugitives and transport federal prisoners. City Marshals provide similar services for city courts.
Constable is a term used primarily in Texas. They are county police who serve legal processes for the courts. They may also contract with other Government Agencies such as school districts, parks and county buildings to provide police services.
Police Officer is a basic term for Law Enforcement Officers. They provide basic police services for cities, universities, airports, and any type of governmental organization.
An agent is usually an investigatory/ detective position with a State or Federal agency that provides special or specific enforcement services. Examples are: FBI, DEA, State Attorney General’s Office and the Dept. of Corrections.