As a newcomer to the series the options for MP are really overwhelming
I tried playing a bit a little while ago and had no idea what I was doing or which playlists to try
I do appreciate the amount of options though. It's just intimidating coming in here and not really understanding anything people are talking about
The most popular classic playlists are probably these two. They are considered the main Halo multiplayer experience:
Big Team Battle - Large team matches. Lots of players (16), larger maps. Can be a variety of gametypes. Good one to start in because you can lag behind a bit and not bring the team down since there are a lot of people.
Team Slayer - The typical 'kill the other team' game mode. Only team-kill gametypes. Half the size (8) of Big Team, takes place on smaller maps. More focused.
The playlists that specify only a game (like Halo 4) are typically a mix of all gametypes and usually have 8 players. SWAT is a mode where you have no shields and everyone dies easily. Snipers is what it sounds like. Doubles is 2v2 only. Rumble-pit is a free for all mode, no teams. Action Sack is the silly mode with crazy abnormal gametypes for fun.
Edit: Oh here is 343's recent ranking of most popular playlists, so you know where the people are flocking to:
1 Big Team Battle
2 Team Slayer
3 Halo 4
4 Team SWAT
5 Team Snipers
6 Halo 3
7 Team Doubles
8 Halo 2: Anniversary
9 Halo 2 Classic
10 Halo: CE
11 HCS
12 Rumble Pit
13 Team Hardcore
Doesnt include action sack since that was added recently.
Explanations of the main gametypes:
Slayer - Your goal is to kill other people. Team Slayer of course means kill the other team only.
Capture the Flag - Team game. Get into the enemy home base, take their flag, take it home to your own base to score.
King of the Hill - There is a marked zone that everyone fights for control of. You get points by spending time in this zone. The zone moves from time to time. Can be team or free for all.
Oddball - There is a skull (the ball) on the map. If you grab it you (or your team) get points for how long you hold on to it. You're marked on the map as long as it's in your hands, so theyre coming for you. Can be team or free for all.
Infected - The zombie mode. One team has enhanced speed but is limited to melee only weapons. The other team has guns and tries to stay alive. If you get killed you join the zombie team.
Assault - It's like reverse Capture the Flag. You have a bomb in your base, you need to take it to the enemy base to plant it. It takes some time to blow and in that time the enemy can disarm it if you give them the chance. Has a few variations. Both teams have bombs, teams take turns planting bombs, or teams fight over one bomb in the center.
There are a couple more gametypes and also there are variations of all gametypes. Typically people separate them into two groups, Slayer (kill people) vs Objective (you have a goal besides killing people). I'm an Objective kinda guy but a lot of people prefer Slayer.