Common nouns are very general terms given to nouns of the same classification. For example, "pen" is what we call the slim, cylindrical devices that release ink onto paper when the pen and the paper are in contact. The nouns Bic, Uniball, Sansa and Parker are names of specific types of pens. The names are borrowed from the company that manufactures the pen. The general word pen is a common noun, and the specific names for various pens are proper nouns.
Common nouns help to establish classes or categories for people, places, things and ideas, and the proper nouns are the individual distinctions. You should think of the common noun as the response to the "what" and the proper noun as the answer to "who" or "which. The common nouns provide English language speakers with a general category for the multitude of things that are named in the language.
Common nouns are created with a lowercase letter at the beginning of the word. This contrasts to the proper nouns, which carry a capital letter at the beginning of the word. There are no other distinctions necessary for executing common nouns. Just remember which is common and which is proper when writing.
Some proper nouns have been use so much that they are mistaken for the general term for the entire category of common nouns. For example, Band-Aid is often used to describe all adhesive bandages used to protect minor cuts. Actually, Band-Aid is a proper noun. It is the name for a brand of adhesive bandages. Despite its common noun-like usage, the word Band-Aid is a proper noun, as are many others that have become household names over the years.