Types of Content
Online games like MMORPGs serve to give players loads of different content to satisfy long-term players. Sure, there's a ton of "filler content" that's repeatable or repetitious, but there usually is a variety of content to choose from. An example of this would be player versus player modes, otherwise just called PvP.
There's also player versus environment modes, or PvE. This is a very general term as you could say anything that's not PvP is considered PvE to many crowds (so long as you are fighting artificial intelligence (AI) in the game or completing an in-game challenge). Regardless of what content you may play, MMORPGs try to focus on players interacting with each other. The most common PvE example would be dungeons and raids - groups of players going through the same instance together and trying to beat the AI. Dungeons and raids are the same principle except raids are generally much harder with a focus on being for very elite players with more complex mechanics to the game.
These are the general rules of MMOs, and each MMO has their own variant of the content and how it's released to players. Sometimes special content will be released for holidays or special events. Guild Wars 2 has three base PvP modes and a lot of different types of casual and hardcore PvE content.
Many MMORPGs support player groups called guilds. Generally, these are the clubs within games where players usually display their guild names and all share a common chat channel for ease of communication. Different MMORPGs have perks for being within a guild beside just having ease of communication - sometimes a shared location called a guild hall that may have in-game vendors or exclusive guild items for guild members.
Within Guild Wars 2, a player may be a part of five guilds. Guilds are customizable to have many roles, each with their own permissions set by the guild leader. A player may "represent" only one guild at a time which means this is the guild you see by their player name. A player may also choose to not represent any guilds while still being a member.
Player Character Development
MMOs revolve around the basic idea of character levels with a maximum level. As you level your character, you unlock more abilities, better gear and access to more maps. Players level by doing content as mentioned above. "Endgame" content is content that is only accessible and doable by elite players who have reached max level, essentially the end of the game. This can include PvE or PvP, depending on the game.
Player character development hinges on the idea that playing the game advances your character. This common goal is what drives most players together to do content for rewards of experience to level and higher level gear.