Difference between Source/Destination

Hello all,

This is probably a really simple question but what is the difference between SourceIN and DestinationIN and the same with the OUT? Both Source and Destination (either In or Out) have the exact same total volume of data that passes through except of course with different IPs. So is SourceIN the amount of data coming into the network and DestinationIN is where it is originating from and vice versa for the OUT (where SourceOUT is the originating point of data coming out and DestinationOUT is where it is going)??

I'm trying to figure out which side of the router some of the interfaces are facing because when I added some devices it doesn't tell me where the Serial interface is (ie, FastEthernet0/0 and FastEthernet0/1 and no serial).

Thanks.