Radar

The radar (also known as the compass) is located in the upper-right corner of the screen. Its position can vary depending on whether you have a panel open and if you have enabled the stretch UI option.

The Radar displays your heading, your field of view, and information about objects around you in the form of colored blips.

Radar blip
If the color of a blip is slightly dimmed, that indicated that the object in question is above or below you. In addition to the colored blips; white, pink and red blips will also show up as a hollow square if the player is in your allegiance.
 * White - A Non Player Killer, a fellow player.
 * Yellow - A Non Player Character, or NPC.
 * Orange - A Creature that can be attacked.
 * Purple - A portal.
 * Blue - A lifestone.
 * Red - A Player Killer.
 * Pink - Formerly used for Sentinels, the pink blip is now used to show a Player Killer-Lite.
 * Green - A member of your Fellowships. A triangle pointing up is the fellowship leader. A triangle pointing down is any other member.
 * Light Blue - Formerly used for Advocates, the light blue blip is now used to show a Turbine-controlled Envoy or any other special character controlled by Turbine employees.

Images
Click image for full size.