Fixed cogs come in two thicknesses: 3/32" and 1/8". As far as I know, single speed cogs are all 3/32".

If you have a 3/32" chain, it will not fit on a 1/8" cog, but a 1/8" chain will fit and work fine with either size of cog.

Pros and cons? 3/32" chains are a little lighter. 1/8" chains may wear a little longer. In practice, there's not much difference.