Battle.net

Battle.net is Blizzard Entertainment's centralized account system for all their online games including World of Warcraft. For the time being the old site is still online for usage at classic.battle.net or eu.blizzard.com/battlenet/en/ (language selector at top-right).

Account merge available
Beginning in spring 2009 World of Warcraft players could merge their original WoW accounts with their Battle.net account. The procedure is as followed, note that it cannot be reversed:

These steps may have changed.
 * 1) Create a Battle.net account
 * 2) Click the "Merge World of Warcraft account"
 * 3) Enter your World of Warcraft details
 * 4) Click the confirm button
 * 5) You now login with your Battle.net account in World of Warcraft.

Required merge
On Oct. 12, 2009, it was announced that WoW accounts would need to be converted into Battle.net accounts within 30 days. This was mandatory, as a Battle.net account would be required to log into the game after Nov. 11, and anyone who wants to start a new WoW account has to make a Battle.net account first. Anyone who had converted their account before Nov. 11 would be mailed in-game a new penguin companion in thanks for converting early, starting later the week of the announcement. The first Unhatched Mr. Chilly companions were mailed to players three days later, on Oct. 15, 2009.

Once your WoW account had been converted, the current expansion that you played at the time would show up as a 'merged game' in your Battle.net account. If your WoW account had an authenticator attached to it, it was also be transferred over.

New Community Sites
On Dec. 4, 2010, it was announced that the World of Warcraft community sites (previously at http://www.worldofwarcraft.com and http://www.wow-europe.com) would now be hosted on Battle.net.

Old Games
If you still have your old Blizzard games, you can add (US, EU) their (CD-)key to your Battle.net account so the both are linked together.

Starcraft II beta opt-in
With the Starcraft II beta Blizzard allowed people with a Battle.net account to opt-in for it. However they required you to have at least 1 Blizzard game linked with your Battle.net account.

Store
Your Battle.net account is directly linked to your Blizzard Store account and purchases should show up on your Battle.net account.