What it looks like
The lower part of the print is fine, then every layer above is offset along one axis. It looks like a staircase. Note which axis it shifted along, since that tells you which belt and motor to check.
Troubleshooting
Layer shifting
Partway up, the print steps sideways and carries on from the wrong place. The printer does not notice, because most printers do not track where the head really is.
Fixes, most likely first
Check belt tension
This is the usual culprit. Prusa names belt tension and loose pulleys as the most common causes, and a belt should feel firm with a little give when you pinch its two sides together. Follow your printer maker's tensioning guide.
Tighten the pulley set screws
Each motor pulley is held by a small set screw, one of which should sit on the flat of the motor shaft. A loose one lets the motor turn while the belt stays put, so the head never reaches the place the printer thinks it is.
Clear the path
With the printer off, move the head and bed by hand. Look for filament wrapped around a pulley, a snagging cable, or a rod that needs cleaning and lubricating.
Slow down
Motors pushed too fast skip steps. You often hear it as a click. Simplify3D suggests halving the speed as a test, and if the shift disappears, speed was the cause.
If it keeps happening
Simplify3D's layer shifting guide also lists motors getting too little current and overheating driver electronics. Those are electrical faults, so contact your printer's maker rather than adjusting the electronics yourself.
Prusa's layer shifting guide covers belt checks for its printers in detail.
Common questions
Yes. If a curled corner or an overhang catches the nozzle, it can push the head off position. Fix warping or curling first, and use Z hop on travel if your slicer offers it.
Something at that position is catching, such as a tight spot on a rod, a cable or a belt tooth. Move the axis by hand through that spot with the printer off.
Usually not. Replace a belt only if it is worn, cracked or will not hold tension; most shifts are fixed by tensioning the existing belt and tightening the pulley.