Words are combined together to form Compound Nouns. Many compound words are used as compound nouns. These nouns very common these days and new such nouns are formed almost daily. Normally these nouns have two parts. The second part identifies the object or person in question where as the first part tells us what kind of object or person it is or what its purpose is.
Police and Man are two different words which can be combined to form another word Police-man or policeman.
Compound nouns often have a meaning that is different from the two separate words. Here are many more compound words which are treated as compound nouns.