Drive by Wire is a relatively new design, and mostly appears in cars which are already technologically advanced. It is a design which frees up space in vehicles which would be taken up by steering columns and throttle cables by replacing them with wiring. Drive by Wire allows car designers more freedom in some areas which analog-style components would otherwise require space for, and also reduces weight a bit for the same reason.
Steer by Wire
This system bypasses the need for analog/mechanical steering by turning the steering wheel into a computer imput device, and the on-board computer outputs that information to the steering system.
Because of the lack of a steering column, there is little preventing the driver from putting the steering wheel exactly where they want it.
In a steer-by-wire design, there is very little feedback in steering from the road/wheels, and it puts a lot of trust in the computer systems linked to them. It can also requires an automated and stronger power steering system or alternative system which can provide some physical movement to change wheel direction.
Throttle by Wire
Normally throttle cables connect the throttle pedal to the throttle body, but the drive-by-wire version of this removes the cable in favor of a computer-controlled version.
Having a digital control over the throttle allows for computer adjustment of the throttle, which means the computer can detect the driver's speed intentions and adjust the throttle flap to compensate for other conditions, and even combine control over that with its control over other engine components to improve fuel efficency.
If anything goes wrong with the electrical circuit, the throttle could be stuck completely closed or open. It also means that if the electric motor that opens and closes the throttle flap loses power, it would not be able to open the flap all the way, or not at all.