The new Hearthstone expansion, entitled Madness at the Darkmoon Faire, will be launching today, presumably at 7pm CET if it follows the usual launch time. The expansion is based on World of Warcraft's own Darkmoon Faire, but this time you will find an even weirder version, as it has been corrupted by the four Old Gods, which are themselves new cards.
A new mechanic is Corrupt. The idea is that "power corrupts", which in this case translates into an improvement to the original card. Imagine a 4/4 minion that costs five mana and has Corrupt. When you cast a higher card (like 6 mana or above), that minion will receive +4/+4, becoming considerably more powerful.
For other details, and one final chance to pre-order the expansion, click right here.