What is it called when a word such as in is at the end of a sentence?

To answer your question, it's known as ending a sentence with a preposition. Another example is "from". Consider the common phrase: "where did you get that from?" "Where did you get that from" is technically grammatically incorrect. The question should be phrased "from where did you get that." It may sound silly, but that's just the way it works. Winston Churchill once said "ending a sentence with a preposition is something up with which I will not put" ("with" is a preposition).