When a wheel is forged, it essentially hammers the wheel into shape under pressure to make a strong, dense construction that is almost always lighter than a cast wheel.

A cast wheel is basically like pouring liquid into a mold and drying. Cheaper to make, but then they are not as durable.

I've seen many Rota's being bent bad and crack, but then since they are so inexpensive, it's not too bad of a trade-off. (IMO)

Edit:

I suggest checking out this guide, well written:

http://www.sportcompactcarweb.com/te...cc_wheelguide/