"Two-handed axes come in many varieties, both single and double-headed. All types slice through foes, felling them as though they were never standing in the first place."— Game guide(src)
Two-Handed Axes are larger-sized two-handed variant of axes introduced in Diablo III. Although some axes required both hands prior to this installment, they were still classified collectively with Axes. Generally, a bladed weapon using its sheer mass is classified as Axe.
Two-Handed Axes are much slower than Two-Handed Swords, but faster than Two-Handed Maces and Polearms. They can be transmogrified into Maces or even Staves (and vice versa), but not into Polearms. All classes except Demon Hunters may use Two-Handed Axes.