Elmo talks in the third person and has a high-pitched voice.
Grover, his best friend, is cuter.
Elmo is not as annoying as Zoe but he's more common. Especially now that they have Elmo's World.

Muppeteer: Kevin Clash

Liz Lot: Before Zoe existed (the good ol' days) Grover was Elmo's best friend. :-P

In Germany, Elmo is called Elma and is female.

Elmo is red. His best friend is Zoe, NOT Grover at present.
His birthday is February 3. He first appeared as a furry little red monster in 1979. He got better known when Kevin Clash started muppeteering him in 1984.

He has twenty minute segment at the end of the show called Elmo's World.And he has a pet fish Dorothy, whom, at the end of every show, at Elmo's world, Supposedly asks a question for all of the kids and Mr. Noodle, (not to mention Mr. Noodle's brother,) to answer.

He has starred in Elmo Saves Christmas, Elmo's Adventures in Grouchland, (1999), and Elmo's Magic Cookbook,(2001).

In 1996 and 1997, more than 5,000,000 Tickle Me Elmos were sold making it the best-selling toy in the world.

Sesame Street was respectible, once upon a time. If you were to watch it you'd learn real-life morality, how to count to 12, and a smattering of Spanish.

Then came Elmo.

Elmo: a saccharin-sweet character designed with the obvious intent of selling toys. Annoying to adults, endearing to childish teenagers, and idolized by children. He doesn't teach anything except how to be a loud, obnoxious hyperactive nuisance with atrocious grammar.

And from Elmo came more marketable characters, and in huge droves. And then they started Elmo's World, where they dedicate a whole 5 minutes to even more inane nothingness, of which most of the time is seemingly taken up by the theme song.

Makes me sick.

Update!

In an even more annoying twist, according to the venerable General Wesc (who we know watches Sesame Street and so he'd know):

[It's no longer] just 5. Elmo's World is a 15 MINUTE segment (with the same one FIVE DAYS IN A ROW and after a week of the same one they change ONE WORD OF IT and play instead.)

This is the last straw. He must be stopped.

J.R.R. Tolkien> The Silmarillion

Elmo was the brother of Elu Thingol (Sindar) and Olwë (Teleri). He lived in Doriath with Thingol and is speculated to be one of the ancestors of Celeborn. He had no parents, at least not in the traditional sense, because he was one of original Elves that awoke at Cuiviénen. By all evidence, he was among the Avari, those who did not make the journey to Aman at the request of the Valar.

                       (Awoke at Cuiviénen)
                                |
                     ___________|___________
                     |          |          |
                  Thingol      Elmo       Olwë
                                |
                             Galadhon
                                |
                         _______|_______
                         |             |
                     Celeborn       Galathil