what those categories mean are as follows....
HC are hydrocarbons, these are unburnt fuel, usually the result of partial ignition or misfires. measured in parts per million.
CO is carbon monoxide, which is the result of partially burnt fuel. your air/fuel ratio affects this the most, richer mixtures produce more CO. CO is highly poisonous, this is sometimes referred to as the "silent killer" because it is odorless, colorless and without a CO-Detector is undetectable.
NO(x) are nitrides of oxygen. these are NOT nitrous oxide, like magazines often call them. these are 1 molecule of nitrogen, with a random number of oxygen molecules attached. these result from too much heat from combustion, which is what happens when you run a to lean A/F ratio. These are non-toxic alone, but contribute to the "green house effect" in that when mixed with sunlight (or more specifically vitamin D) they turn into smog.
now on to your problems.... if you hadn't replaced the cat, i'd recommend looking into that, but long story short, blanco is likely right.