As promised, a literary post. This one is a common misconception. I was disappointed when it appeared on the Big Bang Theory because the show is normally fairly accurate when making its witty jokes. I’m not sure why this one is so persistent when all grammarians agree that this claim is false.
So, what’s a preposition? Well, simply put it’s a word that creates a relationship with another word. Here are a few common ones: for, of, about, in, to, with, on, at, by, after, over, etc.
To use the Big Bang Theory’s example, Dennis Kim the genius Korean boy tells Leonard that his English is pretty good, except for the tendency to end his sentence with a preposition. Leonard replies: “What are you talking about?” (Which ends with a preposition).
Now, is this wrong? Certainly not. To avoid ending with prepositions, one would have to talk like Yoda. It would be “About what are you talking?”.
Apparently this convention for not ending sentences with prepositions began in the 18th century when some grammarians believed that English should follow Latin grammar. Regardless, it is not wrong to do it.
The only time you shouldn’t end with a preposition is when that preposition is extraneous (unnecessary). An example would be “Where are you at?”, where the “at” is unnecessary because “Where are you?” is perfectly proper by itself.