so at launch there were different servers like other MMOs, people could only interact with others on their server. later on Arena.net implemented the megaserver, making all the servers mesh into each other so players could play with anyone playing the game. the only difference servers make now is World Vs World, a large scale pvp mode set across 5 maps. servers are meshed together in groups of 3 and put into matches with 2 other groups of servers. there's a server ranking system which creates tiers in the server ladder. the Meta for WvW changes quite a bit depending on what server / tier you play on. matches on the lower end of the ladder have more, smaller groups and individual players running around, while higher on the ladder you see massive 100-300 player groups running around taking objectives and playing very strategically.