Round Spacers

Round spacers are the most common among spacers which are not threaded and are used primarily in minimum clearance space requirements. They are used to keep two components at a given distance from each other with a screw going through the inside of the spacer into a threaded component, insert or nut. Spacer shapes also come in Hexagon and Square as well.  

There is another product which is almost identical to the spacers which are called Female Standoffs. These do have threads on the inside.