single sided: memory chips on one side
double sided: memory chips on both sides
double sided memory allows for higher densities per module, that is, more RAM chips, higher capacity module. If you insert a double sided module into a single sided dimm socket, you will only be able to access half the capacity. Of course, this assumes it is recognized. It is not as important to note single or double sidedness as it used to be. I cannot say most motherboards use double sided slots, since most motherboards use a mixture of both.
As far as performance goes, it's a relative toss up which is faster. It is not something you will notice unless there is something else going on with memory management.