Smurf

Smurfing is the act of a player with a high rating posing as a player with a low rating.

In Guild Wars, guilds are rated using the ELO rating system and a player from a high-rated guild is able to smurf by playing as a member of a low-rated guild. This can be done using different methods including:
 * Creating a new guild (which starts with a low rating)
 * Using a secondary account in a new or low-ranked guild

Players smurf for various reasons, including:
 * Practicing (for example, trying out new builds) without tarnishing the high-ranked guild's rating or reputation
 * Avoiding the long wait times experienced by high-ranked guilds when being matched
 * Abusing the low rank to lower their opponents ratings

Many people consider smurfing to be unsportsmanlike conduct, and it is forbidden in some games. At least one Guild Wars guild expressly forbids smurfing.

External links:

 * Guild Ladder (on the official Guild Wars wiki)
 * ELO rating system (on wikipedia)