They can either be wood or aluminum. I encourage you to use a wood one. If not, I will make fun of you like I do my good friend Bill Booker at LaSalle-Peru H.S. for using his aluminum one. If you choose the wood fungo, I would recommend taping the barrel. If you hit a good number of fungoes, the tape will increase the durability of the bat. I took the tape off of one once. The barrell was like sawdust, but I re-taped it and it was ready to go. One season, I figured it out, between the high school, American Legion, an junior high seasons I had hit over 10,000 fungoes. Tape it up!
The fungo is used by a coach to hit balls for defense practice. It is lighter weight, so less fatiguing to hit ball after ball. The fungo sheds its weight above the hands. The severely tapered barrel to drops weight and maintains a high moment of inertia. While the bat would be at greater risk of breaking if used in a live batting exercise, it is safe to use in practice because there are much fewer mis-hits.
This is the drill we ran all through high school. It is the most time effective and skill intensive way to warm up a team, but your boys must be able to play catch! Fungo-ers stand in the "fungoe circles" that any good field has (and if it doesn't, you know where they're supposed to be - just outside of the home-plate dirt circle, towards the dugouts). The catch to this method is this - you have two first basemen, allowing both sides of the infield to throw across. Herein lies the only danger - one of the firstbasemen has a lengthy throw back to a shagger, so caution must be excersized (a catcher with a good head on his shoulders really helps).
Fungo bats are, simply put, a long, skinny, and lightweight baseball bat used for fielding practice. Typically, they are around 34 to 37 inches in length and between 17 and 24 ounces in weight, give or take a couple of ounces. They will also almost always feature a 2 1/4 inch barrel diameter to allow for more control during a swing. As you can tell, fungo bats are longer than a normal baseball bat and much more lightweight in comparison. Most fungo bats are going to be made up of ash wood while there are some maple and bamboo wood fungo bats, some composite wood fungo bats, and very few aluminum alloy fungo bats. Almost all of these designs will be under $100 with very few eclipsing that mark.