A T2 deck is a deck that always gets its full setup on T2. As Blazing_Monkey said Bannette is a T2 deck because Shuppet's Ascension let's you search your deck for Banette. I'll give you a run-down on the most common/used T2 decks.
T2B = Start with Shuppet CG's attack Ascension to search your deck for Banette SW and evolve Shuppet. Next turn use a discard supporter/Lunatone GE (nice card to have in the deck) to discard a Banette in your hand and then either do up to 80 damage with ghost head or hit for 80 with Banette's second attack. Rare candy works in this deck.
Quicketune (T2K) = A deck that uses lots of balls and celio's to get four Kricketot in play T1/2. T2 evolve to Kricketune and start hitting for 80. DO NOT USE RARE CANDY IN THIS DECK. Play 4 PlusPower and Stregnth Charm. 4 quickball, 4 celio's, 4 roseanne's and 4 greatball.
T2Doom = A deck that uses Houndoom GE to attack with massive damage T2. Not as good as T2B. Play celio's and pluspowers.