http://www.straightdope.com/columns/read/1694/what-does-it-mean-when-a-patient-is-in-critical-or-serious-conditionAfter checking with a couple of doctors, one on each coast to account for any variation in local definitions, I can state with some certainty that most of these terms don't mean a whole heck of a lot and vary depending on the hospital and the doctor's definition.
"Critical" does seem to mean something fairly standard. As one doc told me, critical condition, by definition, requires care in a critical care or intensive care unit. In general, it's the worst condition (other than, obviously, DOA) a patient can be in, with a high risk of death within the next 24 hours.
Being upgraded to "serious" means there is a lower likelihood of death within 24 hours, but the patient still requires close observation. So, you're definitely thought to be better off in "serious" condition than in "critical" condition, but you're not ready to go run a marathon, or for that matter shuffle to the bathroom.
Another term you've probably heard is "stable," as in, "the patient is in serious but stable condition." This means there are no major active medical issues and the patient's basic vital functions are not fluctuating or in need of support.
There are a host of other terms (you may have heard "good" and "satisfactory" as well, for example), but these are more or less judgment calls without clear definitions. They're basically an official-sounding way to say, "don't worry, he or she will be fine."
Terms like critical, stable, serious, etc., are mostly for the benefit of the media. Most doctors don't use these words when talking with the families of their patients. A detailed discussion of the patient's condition (usually refined to reflect the family's knowledge of medicine and desire to know details) is preferred. Normally, doctors use specific medical terms to describe a patient's problems among themselves.
This whole thing reminds me of the time when I worked at Argonne National Laboratory and there was a minor chemical spill in our lab area. The local paramedic (Argonne has its own fire department) started his own classification of people-- red for right at the spill, yellow for nearby, green for just in the building. Unfortunately, that classification got out into the regular paramedic airwaves, which are monitored by the news media. Normally, I guess, red means practically dead, yellow means severely injured, etc. So all these Chicago-area stations were breaking into their daytime soaps to talk about a chemical spill at Argonne where seven people were critically injured. I was one of those seven, and the worst thing that happened to me was the IV stick!
That's the problem with making up your own classifications--somebody else might not know what you're talking about. One doctor told me that when she was admitting patients to her hospital, she often had some transferred from the critical care unit of a neighbor hospital to the cardiac floor where she worked. Now, the patients didn't get better just because the ambulance or helicopter transferred them over to this new hospital with a different classification system. (But think of the possible cost savings for managed care!) It's just that hospitals have different policies and categories.
When it comes to informing the media about what's going on, the doctors need to use general terms for the quick sound bite, not a detailed description of everything that's wrong with the person. Also, hospitals can't give out details of medical records without the patient's or family's permission. To get around this they can usually say that a patient's condition is critical, serious, etc.
In short, most of these terms, like so much else in our language, are media expressions that have less to do with the reality of a situation and more to do with allowing a talking head to use simple terms to describe a complex situation.