The words are similar, but "grand" has Latin roots. It has the idea of "imposing". "Great" has Germanic roots and is used to refer to non-material things. Material things are often "large" or "big". World War I was called at the time "the Great War".

"Grand" is a French word, and like all things French, it soon gets long in the tooth. So use it sparingly.

It depends on the region. "Great" can mean "impressive" or "important" or "monumental"; however, it can also mean "wonderful." In Ireland, it's common to hear people use "grand" to mean "wonderful" as well, so in this country the words would be interchangeable for both meanings.