Before reading this, you may ask questions like:

What is an MMO? Why would I need to know which game I may or may not like, why can't I just try them all myself?

Firstly, an MMO is a massively multiplayer online game. This means you will be online together will hundreds or thousands of players at the same time, on a single server, playing a game. Why do you need to find a proper game you might ask? There are hundreds or even thousands of different online games out there, many different genres and theme. Most of them are quite similar to each other in terms of general game play. You do not want to play 1 game, find that it is bad, just to hop on to another similar game and waste your hours downloading and trying it.

There are basically a few main types of MMOs. By types, I mean gameplay style, I will not cover the different genres like fantasy or realistic etc, those are up to your preferences when trying an MMO, and are quite obvious.

1) Room-based MMO.

This type of MMO requires you to join a "room" or a "game" after launching it to play it. Meaning, you will be stationed in a lobby looking for player-created games. There are 2 categories of room-based MMO: Dedicated server and Player-to-player (p2p). For dedicated servers, you and your opponents/friends will be connected to 1 single server thus, you will experience less lag. Even if it lags, you all will lag together. For p2p servers, you are connected to the person who hosted the room. The latency will vary from player to player depending on where they come from. Thus in general, dedicated servers will be the type of MMO you want to look for.

Examples of dedicated servers MMO include FPS games like Counter Strike, Team Fortress. Or Casual MMOs like Gunbound or Lands of Chaos Online, League of Legends.

Examples of p2p MMOs include S2 League, DoTA.

2) Consistent World MMO.

This type of MMO is basically hosted by 1 dedicated server. It is 1 big world where everyone logs into it and interact. Such MMOs are generally very common and can be further categorized into Grinder, Non-Grinder, Generic MMOs, Sandbox, Others.

Grinder:

Such MMOs are old-school MMOs where you will recieve minimal quests and all you need to do is to kill monsters repeatedly. Such games include: Maple Story, Lineage1, Flyff, Rose Online, Lineage 2 (old update).

Non-Grinder:

Such MMOs are more focused on other areas of gaming like Player-vs-player (pvp) battles or dungeon raids. Such games will maximal number of quests to help you reach high levels quickly for the end game content. Such games include: World of Warcraft, Guild Wars, Lineage 2 (latest update).

Generic MMOs:

As the MMO market is getting too big and too competitive, producers tend to run out of ideas and copy each other in making new games. Such MMOs that are replica of another, with little or no improvements are generic MMOs. They tend to have very similar gameplay with a slight change of story and graphics. Most of them will give lots of "grinding quests" where you are fooled into thinking there are tons of quests to do. But all of those quests require you to kill hundreds of monsters, forcing you to grind. Meanwhile, the end game content is only for high levels, thus you will be doing the same boring quests over and over again to reach it.

Since most of such MMOs are translated from foreign languages, their updates will tend to be slower. The end game contents will therefore be lesser, making it discouraging for hardcore players.

Such MMOs include: Holic, TwelveSky, TwelveSky 2 etc.

Sandbox:

Such MMOs are unique in a sense, large portion of the game is controlled/maintained by the players. Thus usually a large population is required to make the game entertaining. A sandbox MMO also allows a larger degree of freedom. For example, you can build your own house or castle defenses, steal from other players, drop items upon dying etc.

Examples of these MMOs include: Mortal Online, Second Life.

Others:

These are MMOs that are unique enough to not fall into any of the above categories. They are new and experimental and therefore, not a good type of game to invest lots of money and time into. If the game is not produced by a well funded company, it might even shut down in a few months.

Examples of these MMOs include: Fallen Earth, All Points Bulletin, Age of Conan.

After reading the different types of MMOs, you might ask: How do I choose and know which type of MMOs I will like?

Well, firstly, you have to visit the official website of the game and browse through the forums and browse through youtube for GAMEPLAY videos. It is important to watch gameplay videos and not fooled by the cinematic videos which show nothing about the gameplay itself. Browsing the forums can give you the rough idea on the population of the game and how friendly is the game to new players.

For room-based game, it is important to have a large community or friends to play with. If not, it will be boring waiting in the lobby looking for a "right game" to play in.

For consistent world games, it is important to have a good community (not necessary large, depending on the game and servers). If the community is generally selfish kids, you will not have an enjoyable time playing.

This roughly sums up what to look for in an MMO and what to look out for in them.