While feeding my current obsession with edible landscaping (from Nunavut...), I came across a fascinating page from Designing Edible Gardens on how and why to rotate you vegetable crops. It all finally makes so much sense!
There are 4 phases to crop rotation:
Year 1: Leaves (lettuce, cabbages, etc.). These will feed on high nitrogen levels in the soil
Year 2: Fruits (tomato, peppers, squash, cucumber, etc.). These don't like nitrogen as much, and will feed on phosphorus in the soil
Year 3: Roots (onions, carrots, beets, etc.). The root vegetables will take the last of the phosphorus from the soil
Year 4: Rebuild (peas, beans, etc.). In year 4, you plant vegetables that increase the nutrition of the soil, fixing nitrogen, etc.
There's a lot more to it than this, but it clarified crop rotation quite a bit for me. There is also the option of mixing plants together (one that needs high nitrogen with one that fixes nitrogen in the soil, for example), but that's another matter, and one I'm not yet too familiar with.
On that topic, my copy of Rosalind Creasy's Edible Landscaping has arrived in Vancouver and is waiting for me to come home! 2 more weeks, and I'll get to read all about it and look at the piles of beautiful pictures... So close...