DO: "Dead on." Generally reserved for impressive throws above and beyond the call of duty. Examples:
A bomb that hits a guy in stride.
A fifteen-yard cross into a zone with only a narrow window between defenders.
A useful completion in the face of heavy pressure that would normally be an incompletion.
Generally any throw that can be thrown on a three-step-drop (slants, quick outs, screens of all varieties) is going to receive a "CA" at most.
CA: "Catchable." Run of the mill accurate throws. Encompasses a rather wide range from perfectly thrown short throws to downfield stuff that's a bit off but still feasible to catch. There's a subset of throws that live on the CA/IN boundary, which are...
MA: "Marginal." A slant that takes a guy off his feet, or a seam that is way behind a guy but catchable, or a deep ball that is way short but is a jump-ball situation. These are iffy throws that are still capable of gaining yardage... just not as much as they should.
IN: "Inaccurate." Passes that are plain uncatchable but for extraordinary intervention on the part of the wide receiver. All passes destined for Tacopants, Jason Avant's invisible 11-foot-tall imaginary friend, end up here. Completions can show up here if they're relatively simple throws that require a receiver to make a circus catch. Screens that have their timing disrupted because of a quarterback error also show here, as do open three-yard TE outs that would go for first down but for a late throw. If that should ever occur.
BR: "Bad Read." The grand bull-moose category of quarterback errors, these are throws that should not have been made due to excellent coverage, a lack of awareness of the situation, or inability to locate an open receiver.
UPDATE: Lately, I've started calling things "BRX" for XTREME. These don't get filed separately and are just for discussion of QB performance.
TA: "Throwaway." Passes which are not intended to be caught since the quarterback can find no one open. If the QB clearly has an open receiver that he did not find, these throws are usually slotted in the "Bad Read" category, though they're venial sins compared to throws into coverage. Due to the limitations of television coverage this reclassification is rare, but when there are a lot of these despite the quarterback having good time an eyebrow is officially cocked.
BA: "Batted." Balls which are deflected at the LOS. These are generally regarded as fluke occurences and are not held against the quarterback.
PR: "Pressure." Instances where the quarterback is snowed under before he has an opportunity to make a throw. If he hesitates and does not throw despite being given time, that's either BR or TA. PR is reserved for plays that are blown up because of pass rush, not coverage.
SCR: "Scramble." Instances where a quarterback sees a bunch of running room and takes off to good effect. Usually this has to be an obviously good idea--pick up the first, get eight yards on first down--for it to get filed here. Running for three yards on third and eight will get filed under TA.