Homonyms. Homophones. Words that sound the same but are spelled differently. Words that are spelled the same but mean different things.
He rose and handed her a rose.
Hey, did you give the horse some hay?
To grammar and usage fanatics, misusing these words can drive us crazy. For me it's particularly bad with your/you're and there/they're/their.
Yes, these words can be confusing but c'mon folks, we are smarter than your average primate so let's use words correctly!