from the GW 2 Wiki
Read the full article at https://wiki.guildwars2.com/wiki/World
Mechanics and terminology
Home world: Players choose a "home world" when logging in to the game for the first time; this determines the server you represent during World versus World and your Power of the Mists bonuses. All of your characters are bound to the same world. You can change the home world by paying a transfer fee.
Region: This indicates the location of the data center that stores your account information; it has nothing directly to do with your geographical location. Players from one region cannot enter the same maps as those from any other, e.g. those with NA home worlds cannot play with those based on EU ones.
Players can transfer from NA to EU worlds and vice-versa, but not between China and the EU/NA regions.
Similarly, the EU & NA regions share a Trading Post while the China region has its own.
Megaserver: unlike many other MMOs, all players from the same region are able to play together automatically. When players enter a zone, they are placed into the same instance as everyone else in the same region, until the game determines that the map is getting full, at which point it creates a new instance.
When there are multiple versions of the map, the game attempts to put you in the same map alongside your friends, guildmates, and members of the same world; in the EU region, priority is also given to those using the same supported language, i.e. English, French, German, or Spanish.
To join party members on a different version of the map by right-click their name or portrait in the party UI and choose Join In [zone name]. If the map is full, keep trying until you succeed, as there are always people entering and leaving each instance.
Server: Players often use the term server as a synonym for world, comparing them to servers in other MMOs. However, in Guild Wars 2, map instances are created on a virtual server shared by all worlds. Players can use the /ip chat command to display the IP address of the instance their character is in.
Guesting: Guesting allows you to temporarily change your associated world, thus affecting one of the factors affecting which instance you are placed in. It does not allow you to play for a different team in World versus World or to guest from NA to EU or vice versa. Due to the megaserver system, most players will never need to guest.
To begin guesting, start at the character selection screen and choose World Selection. Click on the Guest button for the world you wish to guest to and confirm your choice. the game will place a suitcase icon symbol next to the guest world.
The next character you play will be guesting on that server and continue to do so until you logout, change characters, or restart the current character. To guest again (or with another character), repeat the same steps. You are eligible to guest as many times as you like, but you may only choose up to two guest worlds every 24 hours.
Caution: Double-check when guesting that you have not accidentally started the transfer process; the buttons are very close together and the prompts are similar.
FYI Maguuma is a medium Pop Server