Hopefully you've read the tutorial on acyclic ethers. If not, go read it.

Cyclic ethers look like this:

They are oxygen containing rings. Since these are very easy to name, they are also referred to by their common names rather than the IUPAC names (although IUPAC naming is still common). Historically, these O-containing rings have been called epoxides. The IUPAC has given the name oxirane to these molecules. Which one you use is strictly up to you. (A random note, the top right molecule is known as tetrahydrofuran (THF))

To name these molecules historically, take the longest chain, name it as you would name an alkene, and then add "oxide" to the end. For instance, for this molecule:

The longest parent chain is a cyclic hexane, or cyclohexane. A cyclo-carbon molecule with double bonds would be called cyclohexene. So, now we add oxide and we get the name cyclohexene oxide. (Note the space).

Cis/Trans conformations

Cis/trans conformations are possibly in cyclic ethers, since the bonds are rigid (no free rotation).

The molecule on the left is trans-2-Butene oxide, while the one of the left is cis-2-Butene oxide.