Your ability with a spell is represented textually by Crawl, but the relative rankings at the bottom end are not very intuitive. Here are the rankings in order:

The 'spell failure' number is compared to 3d100/3 in order to determine whether the spell attempt fails or not. Thus a 'Poor/Fair' spell will succeed roughly 50% of the time, an 'Excellent' spell will succeed 99.9% of the time and a 'Terrible' spell will almost always fail.