Ghasts are only found in the nether. I don't know if they can be created in the over world, but what eves. One of the only ways to kill a ghast, is with a bow. Since they fly, it is like impossible to hit them with a sword.

Ups: they drop gunpowder, but since they fly above lava it usually always burns. They also drop ghast tears.

Downs: they make loud screeching noise that when you spawn in the nether it scares the heck out of you. They also shoot fire balls that can be deflected but destroy all the bricks around you. Try spawning a ghast in your house, see what it does. Also their health regenerates in lava.

Ghast are the one mob that is like impossible to kill. Trust me I tried. Don't go to the nether without bows and arrows. They can also destroy your nether portal, but they can't break stone.