A homonym, for the grammatically and phonetically challenged, is a word that sounds exactly--or almost--like another word. Here are common examples:
to, too, two
there, they're, their
...and a host of others.
And 99.997% of the time it drives me nuts to see someone use the wrong one. Particularly when it's a professional writer--or someone who should be professional--doing it.
One memorable Foxtrot strip shows Jason typing the following sentence:
"Mary hat a lid tell lam."
He hits spellcheck and the computer reports no errors. Well, all the words are spelled correctly. But that's the problem with spellcheck: it's not context-sensitive.
Any dork can write a spell checker. All it does is parse text and compare letters between space characters with a list of words. If the block of letters doesn't match any word in the list, the spell checker flags it as "misspelled". But the flipside of that is that if the word matches any word on the list, it is not flagged even if it's the fricking wrong word.
I don't know if anyone ever wrote a context-sensitive spell checker. I know that Word has a grammar checker which runs at the same time the spell checker does. Unfortunately that grammar checker sucks ass, I always turn it off because it flags things I know are perfectly fine; generally grammar checkers are not well-suited to dealing with fiction, and spell checkers are not well-suited to dealing with science fiction. (Especially if you have a lot of aliens in your stories. I would love to have gotten a look at Niven and Pournelle's "user dictionaries" when they were writing Footfall....)
Whenever I read Knights of the Dinner Table I am incredulous. Jolly Blackburn edited a freaking magazine and he still fills the thing with all kinds of errors. He frequently writes "here, here" when he means "hear, hear", for example.
And don't even get me started on "peddle, pedal". You don't peddle a bicycle, not unless you are trying to sell it. When you use your feet to propel the bicycle via the crank under the seat, you pedal a bicycle. The guy selling hot dogs from a cart on the corner, he is peddling. He's peddling hot dogs; he is not pedaling hot dogs! Damn it! Hot dogs don't have pedals! Fuck!
And while we're on it, damn it, they are not petals. Flowers have petals. Bikes have pedals. (Most spell checkers won't let "petaling" past without a quibble, but I've seen it.)
"Imminent" means something is about to happen. You can't be an "imminent" expert on anything. "My expertise is impending! Watch out! [bam] Okay! I am now the leading expert!" You can have an imminent danger. But if you mean someone is famous, he is eminent.
It's less difficult to have an "eminent danger" but it's not a construction that's typically used.
You can't look at a forum or discussion board on the internet without running into the most commonly misused homonyms, "your, you're", "to, too, two" and "there, they're, their". The misuse of these has become so common I hardly even notice them any more.
I know I can't expect perfect spelling and grammar in forums and message boards and such; and I don't. But when the medium is text, you should try to convey your meaning as clearly as you can. And if you are being paid by someone to write an article, column, or comic, you should make sure your grammar and spelling are correct.
Still, I do expect people to write English, not some pidgin internet-speak. I won't even bother reading someone who substitues "ur" for "your", "u" for "you", "r" for "are", and so on. That kind of shorthand has its place, in text messages and instant messages, but if you are leaving a post in a discussion thread, you're just being lazy. If you can't be bothered to type "you" instead of "u" I'm not going to be bothered trying to wade through your drivel.
The more important the internet becomes to our economy, the more vital literacy is going to become--and it's already vital. I just wish I knew how to fix the problem.