Emotes

Emotes (Emotion codes) are commands which make your character perform movements such as waving, coughing, dancing or pointing. They're an excellent way to convey a feeling to those around you and much quicker with more impact than typing your message in the chat window. WoW supports well over 150 different built-in emotes that can help players immerse even better into the game world especially on the Role Playing (RP) servers.
To perform an Emote, type in the chat window:

/agree
/plead
/shiver

This will perform the Emote and anyone around you will see you. You can also direct your Emote to an individual by selecting the person and then typing /whistle and you will whistle to that particular person. It will also automatically generate "You whistle to <person's name>." Another way to personalise your ingame messages is to type /e first and it will insert your name and then type any message you want. The text will appear in the orange emote text colour so it stands out from the other typed messages.

Making your own emotes will not work for cross-faction communication, someone on the opposing faction would only see "Xinhuan performs some strange actions."

Standard emotes are the only way a player can communicate with players of the opposing faction; they can see the emotes that you perform. On the PvP servers, waving to a player of the opposite faction is a good way to indicate that you do not wish to engage in PvP with them. Or performing the /chicken emote leaves them in doubt they're a cowardy custard.

Emote Listing
Below is a list of emotes in the game. Some of them have associated animations and sound. Most of them have different forms depending on whether you have somebody targeted. For example /angry could generate any of the following 8 depending on who is the target and who is viewing it. Assuming Jack is male and Jill is female.