Sentences begin with a capital letter and end with a punctuation mark. Sentences tell a complete idea or thought. Fragments do not give a complete idea or thought.

Fragment: Toys in the closet.
Sentence: There are toys in the closet.

The first group of words does not have enough information to make sense. It doesn't tell a complete thought, so it is a fragment. The second group of words tells a complete thought.

It can help to read out loud if you're trying to figure out if a group of words is a sentence or a fragment. It can help to think to yourself, "If someone came up to me and said these words, would this make sense? Would I need more information to know what the person was talking about?"