"noun is" =noun's pl. (more than one noun) = nouns sing. pos. (belonging to a noun) = noun's pl. pos. (belonging to more than one noun - i.e. nouns) = nouns' "nouns's" is not a valid construction. Jura's answer is correct, in and of itself, but this the answer to the question I think you are asking.

Does every "noun is" equals "noun's"? ... No. It is almost always better to use "noun is" and not "noun's". This is true of written and verbal English. Also, the meaning of "noun's" can be either 1) a contraction of "noun is", or 2) the possessive (something belonging to noun).

Technically, yes!

Usually noun's means "belonging to noun" -- it shows possession. An exception is its and it's. In this case it's is "it is" and its shows possession. You can use the apostrophe to take out any letter to form a "contraction". This is usually done with vowels. It is not a formal use of English, and really as an English learner you can get by completely without doing this. It really is a way to show laziness in colloquial speech where sometimes the speaker does not want to make the effort to completely enunciate the word. After you are fluent, maybe you would want to do this to sound more "cool" or colloquial, but it really is unnecessary. All you should be concerned with in learning English is how to show possession of a noun: Pete's dog, policeman's gun, Jackie's leg. It gets interesting when you try to show a plural noun possession. Then the apostrophe goes AFTER the s: the doctors' office belongs to more than one doctor, but the doctor's office has only one doctor in it. The weird part of it is that you can only tell the difference when you read it. It will sound exactly the same when it is spoken.

A noun is a naming word for a person, animal, place, abstract idea or thing. Common nouns are boat, dog, cat, love, kitchen, door etc. Proper nouns tell you the name of people and places: Princess Diana, London, etc. For more examples go to http://www.uottawa.ca/academic/arts/writcent/hypergrammar/nouns.html