Chest, front of shoulders, triceps
This is one of the easiest, starting level push up variations out there. Push ups are all about body leverage and since you're starting almost from a vertical position, you're not loading your arms with much weight.
How to do it:
Place your hands on a wall about double shoulder width apart and slightly below shoulder height (where they should remain throughout the movement). Step your feet away from the wall (the further you go, the harder it will get). Breathing in, bend your elbows out the sides so they make approximately 90 degree angles at the bottom of the push up (hands are still slightly below shoulder height). Breathe out and push yourself away from the wall back to your starting position.
Make it easier:
Start with your feet closer to the wall and/or don't bend your elbows as far and/or don't do as many reps.
Make it harder:
Bend your elbows all the way down so your nose touches the wall and/or do more repetitions and/or move to a lower surface, such as a kitchen counter.