[MENTION=38786]Hinode[/MENTION];
There is 2 endings so the developers encourage you to play through it twice.
After you finish the game it just ends, so don't go into the final boss room if you want to do everything!
After you have unlocked every location you are free to roam around.
You can co-op with people by placing your "signature" down for people to come and summon you.
This system is great for getting matched up quickly but if you want to play with friends its terrible since other people might summon your friend before you get a chance to.
There is also NG+ (new game plus) after defeating the final boss you can choose to play the game again with stronger enemies and access to weapons that you couldnt get before and also trophies or achievements.
There are several challenges that people make up to keep the game interesting like the SL 1 play-through and the naked play-through (no armor)
Also you can do a roleplay run where you wear a certain character's equipment and beat the game like that.
Covenants:
You can join different "covenants" or pretty much factions or clans through out the world.
Each covenant is focused on a different role and play style, though some are similar.
like warriors of sunlight is a faith based covenant and it promotes co-op play by making sigs show up more often and you get a special golden sig instead of the white sig.(Characters are also golden, regular is just white)
Warriors of sunlight also get access to lightning miracles. (throw lighting bolts at your enemies!)
another example is the dark wraiths, and their goal to invade people's worlds, kill them and steal their humanity to feed it to the leader for better equipment. The dark wraiths get a very cool skeleton/reaper looking armor.
And the blade of the darkmoon are the people who punish the bad guys. They invade people who has invaded and killed lots of players. When they are summoned they are purple and get accessed to a pretty powerful weapon buff (miracle) and the best talisman in the game when your faith is around 40 (Pretty much best late game but there are better ones when in early game).
There are several more but refer to the wiki for that and further details of these covenants.
Tips and stuffs:
There are lots of secret treasures and areas that you can find just through exploring.
There are tons of shortcuts too so look carefully for those.
Sometimes you might think its a short cut then it might end up to be a trap so be careful
Some areas are pretty dark so it might get scary at times.
enemies might jump out at you too.
There is some areas where its pitch black if you dont have a light spell or a lantern equipped.
Watch out for necromancers, their minions dont die if you dont kill the necro first. (They can get knocked down though)
Dont kill the blacksmiths, they make the game a ton easier for novices.
I've pumped over 100+ hours into Dark souls, pretty addicting game once you get the hang of it.
Dont let the beginning struggles turn you away from the game, keep at it and you will become better.
I've stopped playing now though.
IMPORTANT!!! The game's bosses will get your heart beating and palms sweating so be warned! they are intense at times.
This is an example of role playing (He becomes Solaire when he gets the equipment)
Example of warriors of sunlight golden character and the regular white character and co-op.
gold is the halberd wielding guy and white is Iron Tarkus)
Dark wraith invading example
Hmm anything you still puzzled about?
Is there any post-game stuff in it or is it only pvp crap left after you completed the game? 
RELEASE