Police, or the police force, is a body of men called police officers.
These men have the legal power to enforce the law and to make sure that no one troubles the public and social order.
What's most important is that the police is given a legitimate power to make use of force.

The police force was established as an institution without which our society would turn to chaos.
In our society, the individual property is something most sacred. A sort of social contract binds citizens to respect a set of (more or less) common rules: people are bound to pay due respect to the law of a country.

The police force was obviously implemented in the natural order of things. The motto of the US police forces is: “protect and serve”: protect citizens from mischiefs, and serve what is right.
We can date the rise of the first police institution roughly between the 17th and 18th century. The name is French, because in the aftermaths of the French revolution, the parliament voted for a regulated “police” force.
But seeds of the police as we know it today already grew in Ancient Greece where slaves had the power to guard meetings.

In many countries, the police force is divided into several categories, according to the officers' scope of responsibility. In the United States, two major “types” of police force coexist: the federal police, as represented by the FBI or the CIA and the state police.
The federal police is in charge of investigating federal cases like federal crimes.
Elsewhere in the world, judiciary and criminal forces are also implemented and work hand in hand with detectives.
The state police is subdivided into several branches, among which the Uniform branch or “patrol” that is in charge of responding to emergencies “on the spot”.
Some other branches deal with specific types of crime such as fraud, homicide, computer crime...
Each department is equipped with all sorts of items useful when tough duty calls: SWAT vehicles, teasers, helicopters...

The outfit of a police officer also varies from one country to an other. In the US, UK and France a typical police officer wears a blue uniform, a badge, and many accessories like a handgun, handcuffs, a cap, a baton...
the American officer is portrayed as a lazy dude who loves to patrol handing a warm cup of coffee and a donut.

Police officers may in their duty arrest, search and use of force. Their acts are completely legitimized as long as they remain in the scope of the law. Anyway they have big powers, among which using force. Hence the huge responsibilities they must carry out. Some blunders, like the 1991 taped arrest of Rodney King beat by the Los Angeled police, may happen.

Nowadays, the police officers have several nicknames like bobbies or watchmen in the UK, the heat the fuzz, the cops in the US, and “chicken”, “blue”, “smurf”...