Sorry, I'm sure I saw something like a d and w type o-ring chain, but there are mostly 'O' and 'X' ring chains.

The 'O' ring chain, likes its name looks like an 'o' when viewing it from the side. 
The 'X' ring chain is another type of sealed chain.  When viewing it from the side looks like an 'x'.



The idea behind the 'x' ring is that it has two smaller surfaces that contact the plates and provides less drag then the o-ring chain.  I've read articles that tested the o-ring chains and while they do drag more (not very much) then a non-sealed chain when cold, once the vehicle is run for a few minutes, the chain comes up to operating temperature and it is as good as or better then the non-sealed chain.