What does it look like if you purl every row?
A purl stitch looks just like the back of a knit stitch. If you purl every row, you get a bumpy texture, which is exactly like a knitted garter stitch. Slide the right needle down, and then bring the tip from front to back through the stitch, bringing the yarn with it.
Can you just Purl an entire scarf?
When you choose to purl an entire scarf, you can choose to either purl or knit both sides of your fabric. If you decide to purl both sides, you will come up with a garter stitch, which is the same result if you knit it.