This is the back loop.
This is both loops. You may find that this is often the default place for the hook to go. If a pattern doesn't say anything, this is where the hook goes.
This is the front loop.
Working in the back or front loop only creates a different drape, and allows loops to be left for surface crochet and other more advanced stitches
You don't always use the loops, you can also work into chain spaces. Granny squares for example, work through the space and not into individual loops.
We also can use the stitch post, and make front and back post stitches, they are a little more advanced, but I will cover them soon!