As the story progresses, Eren gains the power of becoming a Titan later identified as Attack Titan (進撃の巨人, Shingeki no Kyojin). Eren uses this power to give Humanity an advantage to defeat their enemies while learning of their true identity and their own history.
Why is it called the attack titan?
Shingeki is a very specific term for akin to a (swift) military advance/charge on the enemy. Using attack oversimplifies this, and in combination with the preposition on and the noun titan (which can also refer to one of Saturns moon by the same name), leads to an error in interpretation.
Whats special about Erens Titan?
His special ability is, that he changes the Titan Shifter himself. He builds willpower. And this willpower strengthens the Attack Titan, which makes him so strong, which makes him so fearless, which makes him so unwilling to give up and which makes him strong enough to overthrow the ones who endanger Freedom.
How did Eren become Attack Titan?
Eren became a Titan by inheritance of his father (the Attack and Founding Titan), and after eating Willy Tyburs sister during the Raid on Liberio he finally gaining the War Hammer Titan.