When you add the apostrophe, followed by the letter "s," all you've done is shortened it from 2 words into 1. There's = There is (sounds different, but means the same) Its the contraction of the word, meaning it now must have an apostrophe. Using this in every day talk is fine, and for elementary level writing reports. However, some will argue for higher academic reports, it is best to use both words instead of the contraction, "there's." I hope this helps.