Added to the end of a word to make a singular possesive (or sometimes a plural possesive with words whose plurals do not end with S, such as people) or a contraction with "is" or "has". Poorly-educated people/foreigners tend to use it for plurals and verbs, but that usage is incorrect.
Bob's cat is black.
Bob's going to the store.
Bob's gone to the store to get his cat some food.
Cat's are fluffy animals.
Bob has a cat that overeat's.