The powertrain of a vehicle includes the source of kinetic energy (usually an internal combustion engine) and the hardware needed to distribute it. In a typical rear-wheel drive vehicle, this includes the engine, transmission, drive shaft/line, and the rear end which could be a live axle with the differential inside of it, or an independent rear suspension design, in which case it would include the differential and axles. Automobile manufacturers typically grant a different (and longer) warranty on the powertrain than the rest of the vehicle.
Compare to drivetrain, which includes all of these components except for the engine, and deals solely with power distribution.
In modern automobiles the powertrain is managed by a series of computers including but not limited to the PCM or powertrain control module, ABS (antilock braking system) computer, transmission computer in the case of vehicles with an automatic transmission, and possibly a traction control system. Most modern vehicles are front wheel drive and use a transverse-mounted engine coupled to a transaxle.