forged or cast iron

[im4abj]

whats the difference and whats better?

[MrPbody]

Cast iron is molten iron "poured" into a mold. "Forged" steel is a much "cleaner" alloy, roughly shaped, heated to near molten temps, and "hammered" or "forged" into final shape, before machining begins.
For certain applications, cast iron is quite good. It has a very rigid property that can maintain shape and size over a broad range of temperatures and load conditions. Until the recent (last 20 years) requirements of maximum weight reduction in production cars, cast iron was the material of choice for engine blocks and most cylinder heads. Today, we see mostly aluminum.
A forged steel crankshaft is considered the ultimate in strength (particularly modern "non-twist" forgings). Contrary to popular belief, a forged crank is stronger than a "billet" (carved out of a contiguous piece). The latter is more popular among racers, as you can get virtually any stroke/journal combination you desire, and still have plenty of strength. With a forging, there are limits to the amount of material that can be safely removed, depending on the size and purpose of the original forging.
Another area where the crossing continues, are connecting rods. Chevy, Ford, Olds, Dodge, most Japanese and European makers supply forged rods. Older American engines may have had cast rods. Pontiac is the most notable, as the cast rod is the weakest link in that chain. Buick and Caddy also used cast rods in their big engines. They too, have the rod as the weak link, but are seldom pushed beyond the functional RPM range, unlike the Pontiac.
What particular parts are you asking this question about?