It's comprised of the crankset, chain, front and rear derailleurs and pedals.
The Chain Tool: An Essential Take-Along For Off-Road Riding - Because a broken chain can quickly ruin a ride and force a long walk home, the most valuable tool in your seat pack when you're trail biking might just be the chain tool. The chain on an off-road rig is susceptible to damage due to the excess wear dirt puts on the links and the greater opportunity for foreign objects like sticks and rocks to beat on the chain. But, as long as you're packing your chain tool (they're small and inexpensive), should your chain separate or a link get damaged, you can simply remove the wrecked link and rejoin the chain to continue riding. Remember though, that the chain is now shorter so avoid using your large chainring in combination with the larger cogs on your rear sprockets because this can jam the drivetrain. Also, if your chain requires a special replacement pin for repairs, as many do, always carry a spare or two in your seat bag (tape them to your chain tool so you don't lose them).