In crochet, it's a convention that at the beginning of a row or round, where you need to get to the height of the next row, you chain a certain amount of stitches to replace the stitch. The relationship goes something like this:
Sl st- 0 chains
Single Crochet - 1 chains
Half Double Crochet - 2 chains
Double Crochet - 3 chains
Treble Crochet - 4 chains
Double Treble Crochet - 5 chains
... and so on a soforth.
The key is, you can not only substitute chains for a stitch, but you can substitute stitches for chains.
Take this granny square. It's pretty standard, but you see where the single crochets are (hint: The red x's) They are worked into the 3rd chain. I do this instead of doing a slip stitch to finish off the row.
Let's see if I can explain it with pictures.
Things making more sense now? Let's compare the two different types of Granny Squares.