Single Ranked Ram vs. Dual ranked

[cyrusm14]

I have an Assus GL553VD laptop. It has two slots of ram and one slot has 8gb in it. I am wondering do I need to buy another 8gb of single ranked ram or dual ranked. I am not sure what either are. thank you in advanced.

 

[cyrusm14]

MERGED QUESTION
Question from cyrusm14 : "Single Ranked and Dual Ranked Ram"

I have an open slot of ram in my laptop, do I need single ranked or dual ranked Ram Sticks. It currently has 8gb. I would like 16 but I'm confused if I should buy the dual ranked or single.