Knitting: What is Front and Back Loop?

While the Knit A Block A Week series continues, I came across some patterns that specifically ask for knitting in back loop.

I have seen my mother knit for many years and learnt knitting by noticing her knit sweaters, scarves, etc.
For the matter of fact that knitting is commonly done in one way, that is, knitting in front loop, be it knit or purl (at least that's was what I thought); I never knew that there was a 'functional' back loop also.
Now that I am learning more about the different stitches, it is interesting to know that various types of stitches can be formed by stitching in these loops.  

Front loop is the one that is directly in front of us and the one that is farther and behind the needle is the back loop.

There is another component that cannot be ignored when talking about the front and back loops and that is the Horizontal Bar. It is the bar or strand present between two stitches.

Happy Knitting!!!