You basically create a server yourself. Either a private one with a password, or a public server. Once you log out of it (and all others do as well) that server cease to exist. Then you can create a new one or join someone else's. Think of it as a RTS session. It only exists as long as the players in it exists. You can have up to (correct me if I'm wrong, been a while since I've done big runs) 16 players in one session.
The progress of the character who creates the session dictates what's open and available, but if someone further along joins the session and travels to higher areas in the act... Well, he can open portals the original player can go through and thus skip content if he/she so wishes. A common way to level up fast is to get a friend to port you along to all key areas up until Baal. Then the original player can just join Baalruns for xp and some loot.
To compare gear you join sessions with whoever you want to "inspect".
You can carry characters from Singleplayer to Multiplayer in LANs, but I seem to recall BNet characters are BNet chars only. Be aware that after (again, correct me if I'm wrong) about two months of inactivity, your BNet character is wiped.
So, newbie info:
1. Do away with the first two/three acts quickly. Then get a friendly soul to port you quickly up to Baal.
2. Do NOT join in on a Baalkill. Only kill his minions before he runs into his portal for xp and loot.
3. Solo Mephisto. Most classes (especially Sorc) can do Meph really quickly and safely. This will net you loot.
4. Never throw away/sell Runes. Some of the Runewords are really powerful.
5. Never kill the Cow King in the Cow Level. This will close the Cow Level permanently.
6. Try to find a working Maphack. It's technically cheating, but everyone and their grandmother use it.
7. Get friends. Especially friends with high level Sorcs or Javazons. You won't regret it when they'll run you through Cow Level and Baal repeatedly. Best xp/loot you'll ever see.
8. When you have farmed enough Baalruns for xp and levels you can kill him. Make sure you've gotten some Resistance stuff by then, and have decent gear. That way you'll be able to just run through the first few acts in a very short time to get up to Baal once more.
9. The game doesn't really kick off gear or character wise until you've gotten to the hardest difficulty.
If you have any questions, just come here and ask.
Runewords page When you hit the second difficulty level, farm Enigma runes (Jah, Ith, Ber). Most classes will benefit alot from it that early on, and getting Teleport will really speed up your progress.
Edit:
Also, if you are curious about how various classes should be built for best performance, just ask. I've built most classes at some point, so I think I can remember most of what you need to consider before putting points anywhere. Remember, a used skillpoint is gone forever and you can't rebuild without rerolling.
Edited, Jul 23rd 2008 6:43am by NorthAI