Today's bats are made from aluminum or composite.
There are several different grades of aluminum. Lower price point bats are made with lower grade aluminum and are great for beginning players or recreational players.
If you're playing at a more competitive level, you're probably going to want a bat made with a high performance alloy. A high performance alloy will allow the bat to have a longer barrel, a bigger drop weight and lower MOI (or faster swing speed).
Bats made from Composite materials are usually the best performing. Composite materials are usually lighter than aluminum and can be engineered to maximize both barrel length (sweet spot), and the swing weight.