I've heard of at least a few different grades of fertilizer:
Are these standards? Where are they defined? What's the difference between each (and any other grades I missed)?
Edit: I assume most of these have something to do with the levels of purity and contaminants, and I've read the horticultural grade does, but I haven't heard anything on that for the others. As you see in the link above, one person also says that horticultural grade and agricultural grade have to do with size of granules and the number of micronutrients. Other than that, I'm not sure what to think of the rest of these.