@Moltovir: there are many things you should know about D2, I'll try to summarise the most important ones, and answer you questions at the same time.
1. D2 (did you buy the expansion pack Lord of Destruction too? I hope so, it is much more fun with the explansion) can be played in single player locally, on local networks and online, at Battlenet,
http://www.battle.net. The game is best when played on Battlenet, since there are certain features that are only available online. To play at Battlenet (Bnet for short), you just need to install D2 on a computer with internet connection and log on to Bnet. You create an account with username, password and your CD key, and then you can build you characters and play just the same as locally. Bnet characters cannot only be played at Bnet, you can't play them both online and locally unless you play at Open Battlenet, but that's nothing I recommend since most people there played with self-made hacked items and characters.
On Bnet, there is something called Ladder season. The Ladder is a closed environment. Blizzard reset the ladder 1-2 times a year, and then the characters who played on the Ladder will become non ladder, and new characters must be made in order to play Ladder. When you start a new character, you can choose if you want to make it ladder or not. I always play ladder since Blizzard introduce new items or features on every new ladder.
2. If you play on local network or online at Bnet, you can play with others. In a game, there may be up to 8 characters, so you can team up with friends or strangers to make a party. Party play is more fun, and also makes the gameplay easier. On Bnet, you can create open games (ie games without password) or join others' open games. The disadvantage with playing in open games is that many people just want to rush their chars to higher level as soon as possible, or make item finding rushes, so they play only parts of the game. You can try looking for games named something with "quest" or "all quests", for instance "Act1allquests", that usually denotes the group play through the entire game.
You can also create closed games (like I and my friends do) so only those you give the password to, can enter your game.
You can party with other players and battle monsters, or you can play PvP, player versus player, and battle other players. In the game, you can go hostile on a player, and then you can duel. Duelling is a very common form of gameplay, many people team up in clans and duel with each other in gangs (so called "clan wars").
3. A "build" is a character plan. Usually the builds get names depending on what skills they use, but sometimes also after certain items they are meant to use. A Bowazon is an Amazon that uses a bow, a Strafezon is a Bowazon who has Strafe as her main skill. It is important that you play your character build well, otherwise it will be impossible to get all the way to Hell difficulty (first you play the game at Normal difficult, then you start again from the beginnin at Nightmare difficulty, and finally you start once again from the beginning at Hell difficulty. When you beat Baal at Hell difficulty, you have beaten the game.) D2 is very much about making builds that make it passed Hell Baal, and items for that build.
Necros can be played as Golemancers, Skelemancers, P&B (poison and bone) and various combinations. If you play alone, you must build a character that is viable on his own, if you play in a party you can specialise. Maxing Bone Spear like Lonely Pilgrim suggests in one way of playing, but you will need more than that to make it through Hell difficulty.
