Yes Lil' Lori, that's exactly right; you go up the Gatun locks to Lake Gatun, cruise Lake Gatun, back down the same locks, and back to sea. You miss the very narrow Gaillard Cut, the Pedro Miguel and Miraflores locks, and the thrill of sailing under the Bridge of the Americas.
The 10 day gives you a good look at the Canal and its operation; but the full transit is really the way to experience it. As cousins noted, the full transit cruises have more sea days than most cruises--as they are essentially repositioning cruises; but to me, that's a plus--I love lots of long leisurely days at sea--but not everybody feels that way.
But Lori, you sound like you have the right attitude and would really love the full transit. I did it in 2000, I'm doing it again next April, and hope to do it again and again. I love it the way some people love Alaska; not that I have anything against Alaska--I just prefer long warm weather cruises.