Ingrid's right, it's just taking the working yarn from the back (for knit), and shifting it BETWEEN (cannot emphasize that enough) to the front to purl. Then insert your right needle into the front of the loop on the left needle, and wrap the working yarn counterclockwise around the right needle tip and pull it out. Voila! Shift it to the back to knit.
I learned to knit from a book, but was stuck for TWO YEARS!! (AAARRGGHH

) doing garter stitch, 'cause the dang books (yes, more than one) omitted this very simple but CRUCIAL step, while going on to teach yarn-overs, cables, etc. Unbelievable. Maybe they take it for granted that you'll get it, but I had to go to a knitting clinic to ask the yarn 'Dr." for help at my local Michael's. Once learned, I was THEN well on my way to cables, lace, etc...
