The arakh is the standard weapon used by Dothraki warriors. It is a long blade, making a semi-circular curve in its lower half, making it half-sword and half-scythe.
Jorah Mormont notes that while the sweeping blade is effective and indeed necessary for a warrior on horseback, the arakh pales in comparison to the Westerosi double-edged sword for battling without a mount or at penetrating armor.
Although a Dothraki weapon, it is occasionally used by non-Dothraki, such as the mercenary captain Daario Naharis.
In the books
The arakh is similarly described in the books. The weapon overall is an original invention of GRRM, but does bear some similarity to ancient "sickle-swords" such as the khopesh and the falx.
According to the TV series official pronunciation guide developed for the cast and crew, "Arakh" is pronounced "Ah-rock", as opposed to "Ah-RACK".