What it looks like
Instead of a smooth wall, you see lines or ridges running around the print, often at a regular spacing. A single odd line is usually one bad layer. A repeating pattern points to something cyclical in the printer.
Troubleshooting
Z banding
Horizontal ridges that repeat up the walls of a print. The spacing of the bands tells you where to look.
Fixes, most likely first
Check the Z axis
With the printer off, turn the lead screws by hand or move the Z axis through its full travel. It should move smoothly with no tight spots. Clean and grease the lead screws the way your printer's maker describes, and check the couplers are tight.
Stop the bed wobbling
On a printer with a moving bed, a loose bed or worn wheels lets the nozzle height vary layer to layer. Adjust the wheels or eccentric nuts until the bed has no play.
Steady the hotend temperature
Watch the nozzle temperature during a print. Simplify3D's guide says a well-tuned printer holds within about 2 °C. If it swings more, run the PID tune your printer maker describes.
Try another filament
Filament that varies in diameter pushes out more plastic in some layers than others. If a different spool prints smooth walls, the filament was the cause.
Read the spacing
Measure the distance between bands. If they repeat at the same spacing as one turn of the Z lead screw, the Z axis is the likely cause. Irregular bands point more toward temperature or filament.
Simplify3D's lines on the side of a print guide covers the same three causes.
Common questions
Z wobble is one cause of banding, from a lead screw or coupler that is bent or misaligned. Banding can also come from temperature swings or uneven filament.
Only if the lead screw is the cause. Work through the checks above first, because a new part does nothing for a temperature swing or a bad spool.
A wobble in one direction marks the walls facing that way most. That also points to the Z axis rather than the filament.