prmtvr :
The question says it all.
ddr3l is ddr3 memory but has much lower voltages, (eg.1.35v & 1.25v)
The new 3rd gen intel core processors(ivybridge) support ddr3l.
lower voltages mean less heat and will increase the life of your module.
ddr3l is usuall $5-$10 more than ddr3