When you reach level's 7 and 14, you'll be given a specialization point. This adds to your class and gives you some unique abilities. Each class has four unique specializations that you can unlock. Yes, you have to unlock them first before you can use them. You can either unlock them by speaking with your allies or by purchasing manuals from merchants. Learning any one of these specializations will unlock this achievements. You can only select specializations when you level up.
Assassin - Learned from Zevran or purchased from Alarith's Store during "Landsmeet"
Bard - Learned from Leliana or purchased from Alimar in Orzammar
Duelist - Learned from Isabella in the Pearl in Denerim
Ranger - Purchased from Bodahn's Wares in your Party Camp
Arcane Warrior - Learned from The Presence in the Brecilian Ruins during "Nature of the Beast"
Blood Mage - Learned from the Desire Demon in the Fade during "The Arl of Redcliffe"
Shapeshifter - Learned from Morrigan or purchased from Varathorn in the Dalish Camp
Spirit Healer - Learned from Wynne or from the Wonders of Thedas during "Landsmeet"
Champion - Learned from Arl Eamon after "The Urn of Sacred Ashes"
Templar - Learned from Alistair or purchased from Bodhain's Wares in your Party Camp
Berserker - Learned from Oghren or purchased from Gorim in Denerim Market
Reaver - Learned from Kolgrim in the Wyrmling Lair