The mini-map gives you an overview

The mini-map gives you an overview of your current area and appears by default in the upper right corner of your screen. With it, you can track the locations of allies, enemies, NPCs, and other players. You can also use the mini-map to communicate strategy with your teammates by clicking the mini-map to "ping" that area or by left-clicking and dragging the mouse to draw on the mini-map. Drawing is especially useful when coordinating your group's movements and strategy, or when you're leading a less experienced team of allies through a mission or quest with which you are familiar.
Allies:
Green dots represent your allies.

Players:
Blue dots represent other online players inside the confines of a town. When they are on your team, players become allies (green dots).

Foes:
Red dots indicate the location of foes.

NPCs:
Yellow dots represent non-player characters (such as villagers, traders, and henchmen) who are not hostile to your party.

Danger Zone:
This circle represents the radius beyond which monsters cannot detect you. Monsters inside this "aggro" zone will become aggressive and attack you.

Ping:
Click to "ping" the mini-map and point out a location for party members.

Mission Pings:
During missions, red pings often indicate high-priority targets. Green pings indicate the location of important items.

Map Drawing:
Click and drag to draw on the mini-map to diagram strategy for your allies. Opponents in PvP cannot see your side's mini-map drawings. http://www.guildwars.com/gameplay/exploring/gettingaround/