hi
i want to buy world of war craft so i want to ask is it worth the $15 a month.
is there pvp and trading With other players plus there ar eso many packs for the game what do i buy i just want the core game. and are many people still playing
can i talk ot any player in the game?

Everything the other guy said is pretty much right on. I might add that you play for free with certain limitations up to level 20 for an unlimited amount of time.
The trial free trial version doesn't let you trade, auction or join guilds but if you like it you pay the $20 to upgrade to a full account of WoW Original and the first expansion, Burning Crusade. This will let you get up to level 60 or 70.
Then comes the second expansion, Wrath of the Lich King, which will get you up to level 80.
Then comes the third expansion, Cataclysm, which will get you up to level 85. The current cap.
You don't need all the expansions to play the game. I just bought them as got to that part of the game.

With a full account you can trade with other people, use the Auction House (the game with-in the game) and form and join Guilds.

There are servers that let you PvP, PvE or Roll Play. Even on a PvE server you can flag yourself for PvP.

There is a running chat at the bottom of the screen so you can talk to other players and it is good entertainment as well. There is also an in-game voice chat set up and several third party programs so can actually talk to people, which is a must for the high end Raiding and Arena teams.

For PvP, all the servers have certain areas that are for PvP or you get in the Battle Ground queue to go to one of the Battle Grounds and play things such as capture the flag, capture and hold various territories and storm a fortress and such. There are also Arena teams that battle each other but I am not into that myself yet.

Ok, Here we go.
Yes to PVP, in fact there are realms that are PVE and PVP realms.
Yes, You trade with other players and talk to those players on your realm if they are on your faction. If you want to talk to the other faction you just make a character on that faction to talk to them too.
If you buy the Battle Chest, which includes wow and the first expansion, you can go up to lvl 70.
Wrath of the lich king is lvl 80, and Cataclysm is lvl 90.
If you want to be a Goblin or Worgen you need Cataclysm, and you have to buy each game up to Cataclysm, like.. you cant buy the first wow game.. and then cataclysm, you need to buy them in order.

WoW currently has around 10-11 million Players.

I recommend buying the Battle Chest for around 20 Dollars, and see if you like it, it includes one month of free time too.