Prolog

Prolog is a logic programming language especially useful for artificial intelligence related purposes. Even though it is rather old (more than 40 years old) it is still used today.
A Prolog program is made of declarative statements named clauses, and is run by executing a query.

Clauses

There are two types of clause: facts and rules.

Facts

A fact is a statement which is always true. For example, "Yumemi is a company" is a fact. In Prolog, this could be written like this:

yumemi.prolog

isCompany(yumemi).

In this clause, yumemi is called an atom and can be thought of as a constant.

Rules

A rule contains a head and a body. It is used to infer new facts from existing ones. Here is an example:

car.prolog

hasWheels(X) :- car(X).

Here X is a variable. It can be replaced by any atom as long as it's the same on both sides. This rule reads "X has wheels if X is a car".

Queries

A query is simply a question asked to the system, using the same syntax as for the clauses. Let's take a simple prolog program:

weapon.prolog

has(link, sword).
isWeapon(sword).
hasWeapon(A) :- has(A, B), isWeapon(B). 

Live example
One can ask : "Does Link have a weapon ?" like this:

hasWeapon(link).

The output will be:

yes

One can also ask:

hasWeapon(X).

which means, "Who has a weapon ?" and get the following result:

X = link
yes

How does it work ?

When getting the query

hasWeapon(link).

the system looks for a rule which gives such a result. The first one which matches is

weapon.prolog

hasWeapon(A) :- has(A, B), isWeapon(B). 

with A = link.
For hasWeapon(link) to be true, has(link, B) and isWeapon(B) need to be true.
First, the engine tries to validate has(link, B), which is true for the fact

weapon.prolog

has(link, sword).

so B becomes sword.
Then, isWeapon(sword) is true because the program contains exactly this fact.

Conclusion: has(link, B) and isWeapon(B) are true, so hasWeapon(link) is true.