RPG or RP or RPing - All forms of "roleplay" or "roleplaying game".  A type of game where a player assumes the identity of one or more characters, and pretends to be that character, either through writing or acting.  (In this case, writing.)

God moding - Making one's character unreaslitcially powerful, or able to succeed in anything, or being invincible.  Basically any sort of play that seems god-like.

Power Player or Power Gamer - Someone who god-modes, basically.  They always play powerful or influential characters, and often steamroller over other players.

Metagaming - Using OOC information ICly...such as having your character know something that he or she should not know.

OOC - "Out of Character"

IC - "In character"

Tag - As in "tag: Dumbledore"....  A "tag" is a notification that a particular character has been addressed or needs to react in some way to the post, often put in the subject line.

Attn: - Used in the same way as "tag".

PG - "Permission Given" or "Permission Granted" - used when you want to let another player know they have permission to assume something of your character, or control the character in some way.

Fade to Black - A term used when you are getting into a sexual scene, and don't want to (or can't depending on the age level of the game) write the explicit parts.  You lead up to it, then end with "fade to black".

GM  - Game Master, the person in charge of the game and plots and characters...etc.  Sometimes called an "ST (storyteller) of DM (dungeon master) depending on what you're playing.

Mod - In the case of E-Mail RPing, a Mod and a GM are usually the same thing.  Mods don't always have to be involved in plot, though.  They're the ones who maintain the list and/or the game.

NPC - "Non-Player-Character"  A character that is controlled by the GMs (or players when given permission) used for furthering plot, keeping players' character occupied, or setting the scene.

PC - "Player Character."  A character that has a real player.  (Opposite of NPC.)   GMs are allowed to have PCs, too.  Can be differentiated from "personal computer" by the usage.  Thus: "My PC just killed five Death Eaters".  It probably does not refer to a home computer.  (Hee hee.)

OT - "Off Topic"  Anyhting not related to the main focus ot the list or thread.  In the case of E-Mail RPG, it is usually synonymous with "OOC".

PbEM - "Play-by-E-Mail"  The kind of RPG this is.

Tips:

 Know the world: When you play in any specific genre, you should always have a good grasp of the world you're trying to roleplay in.  You don't have to be an expert, but you should be familiar with the mood and the mechanics of the genre.

Along with knowing the genre, you should be familiar with the game itself.  You want to know the time period of the game, the rules, and what the GMs are looking for.  Don't be afraid to ask questions.

Learn proper spelling and grammar: Yes, this is an important part of roleplaying.  Take it from the former Typo King...a blob of bad grammar, netspeak, horrible spelling/typing, absent punctuation, etc., turns what might have otherwise been an enjoyable post into a tedious deciphering session. 

Give Players a Chance - When posting in a scene, make sure you give all the players and characters in that scene with you a fair chance to respond...don't leave people behind.

Description and mood: This is a roleplaying game, but it is also a story.  One word posts are not very fun to respond to on a regular basis, and makes it seem as if the player is not very interested in the game.  Don't be afraid to be a little descriptive. Describe how the character feels, what he thinks.  His surroundings, how the person perceives his surroundings. 

Know thy neighbor - It helps to know the people you're playing with.  Don't be afraid to ask for IM names, and get to know what they're like.  You might make a friend! 

Know thyself! - This seems to be a "duh" sort of thing, but you'd be surprised.  When you pick up a canon, or even make a custom, you should know that character well.  Granted some things come out in roleplay, that's the nature of roleplaying.  But you should have a firm idea of who you're playing.  People who intimately know their characters can easily write for them and know how they'd react in any given situation.  A good characterization technique is to go online and find a personality test...and take it as your character.  It's fun, too!

Plot - Especially in a freeform, E-Mail based RPG, not all the plotting has to be done by the GMs!  Player-driven plots, both good and evil, are what make these game go 'round.  Don't be afraid to get together with other players and plot stories between your characters, or go to the GMs for ideas of where to take the game or things you'd like to do.

Have fun! - This is so important that it is also in the rules.  If you take the game too seriously, you will kill your own (and others') enjoyment of the game.  If you squabble over little things, you won't enjoy yourself. Remember that the "G" in "RPG" stands for "game".  Games are played for fun!