Anatomy of a Crochet Stitch
Viewed from above, you can see that a crochet stitch looks like a lying letter V, like ≻ or that ≺. In most cases, you insert the hook under both loops that build this V-shape to crochet a new stitch. This spot right underneath both loops is called the "top" of the stitch. Nevertheless, it may be useful to work only into one loop or into the back bar behind them. In the photo you can see the parts of a stitch you can crochet into.
The front loop is the loop that is closer to you. In crochet patterns you will find instructions like through front loop only (e.g. sc-tfl) in case you have to work your stitch just into this loop.
The back loop is the one that is farthest from you. In case you have to work just into this loop, your crochet pattern calls for through back loop only (e.g. sc-tbl).
It's very useful if you want so save the front loop to add an additional row of stitches on the surface of your crochet piece later, or to create a little nook on a 3-dimensional crochet piece. The line of front loops that remains if you crochet only into the back loops, gives you also a clear separation e.g. between rows of different color.
BACK BAR (BACK BUMP)
The back bar, or back bump, lies right behind the back loop of a stitch. Working into this spot leaves you with a visible line of stitches (V's) on the surface of your fabric. You can use it also to create a well defined 90° nook.
distinguish the sides of a crochet piece
For various reasons it can be important to distinguish what's the right side (front) or wrong side (back) of your crochet (e.g. if a pattern calls to work only into front or back loop of a stitch). In the following photos you can see the right and wrong side of single crochet stitches worked in rounds or in rows.
single crochet stitches in ROUNDS
single crochet stitches in ROWS
Right Side (front side)
Here's what the right side (front side) looks like if you work single crochet stitches in rounds. You can spot little V's on the surface.
Wrong Side (back side)
And that's what the wrong side (back side) looks like if you single crochet in rounds. You can easily identify this side by the horizontal lines of the stitche's back bumps.
similar front and back
This sample shows single crochet stitches worked in rows. The front and back side look alike. You can find the V's and also the back bumps on both sides.
The position makes the difference
Usually, the wrong side (back) of a three-dimensional crochet piece is inside, while the right side is visible on the outside.
If you imagine the edge of your crochet piece as a clock, you crochet on 6 o'clock and always insert the hook into the stitch from the outside inwards. You crochet clockwise from right to left. The right side of the fabric is visible outside and the wrong side will be inside.
CROCHETING ANTI-CLOCKWISE / COUNTER-CLOCKWISE
Sometimes it is necessary that the wrong side is visible at the outside of your crochet piece (e.g. for bobble stitches that build their bobbles on the back). To achieve this, your crochet position is 12 o'clock. You insert the hook from inside towards outside. That way you get the back side of your fabric visible outside while the right side is inside your piece of crochet. Even on this position you work from the right to the left, thus makes you moving anti-clockwise.