A standoff is a threaded separator of defined length used to raise one assembly above another. They are usually round or hex (for wrench tightening), often made of brass come in male-female or female-female forms.
In electronics they are frequently used to raise a printed-circuit board above a surface. Insulating standoffs keep two parts from touching each other, thereby preventing electrical shorts. When used to fasten cable connectors (i.e. D connectors) together, they are called jack screws.
In contrast, a spacer is an un threaded piece of tubing which lets the entire bolt pass through. Since they cannot be tightened, they are usually round
Brass hexagonal spacer with male screw / PCB standoff spacer
Spacers / Stand off
Hex Spacer - Female to Female Thread
M3 x 6 mm
M3 x 8 mm
M3 x 10 mm
M3 x 12 mm
M3 x 15 mm
M3 x 20 mm
M3 x 25 mm
Typical Usage of Standoff/Spacer in Electronic Board Mounting, Electronic assembly, computer, computer cases and cabinet