Spelling or grammar error?

I know that it may sound tautological, but the defining difference between two different kinds of error is the kind of error: It depends on whether the writer's mistake was in believing that they were using the correct spelling, or the correct grammar.

For example, if the writer wrote "You're dog ate my homework!" in the honest belief that "You are dog ate my homework" was a grammatically correct sentence, then the error is one of grammar - but if they had written that thinking that "You're" was the correct spelling of the possessive, then the error is one of spelling.

So, it's not really possible to know what kind of error it is without asking the author what they meant - but I suspect in most cases it's not important to know what kind of error has been made, so long as you know how to correct it.

(Personally, I reckon that most misuses of "your" and "you're" are spelling mistakes, since they're homonyms and most English speakers I know would be able to expand the sentence correctly, but that's just my experience talking. Other people's experiences may differ.)