A carbonyl is a molecule containing the C=O functional group. For the purposes of this chapter they will be split into 2 categories - one category will be ketones and aldehydes, the other will include pretty much everything else.

To see how this breaks down;

Of the latter class a couple of examples have been given - some other important carbonyls from this group are shown below;

The reason for splitting the carbonyls up like this is that they display 2 different reactivity types - you may have noticed that the second class differs from the first class in that they all have some sort of leaving group, whereas the ketones/aldehydes do not. Because of their LG, the second class can undergo nucleophilic substitutions, which were referred to here. This general reaction is pictured below, with the leaving group demoted X;

In some cases this will require perhaps acid or base catalysis to make it go - an example of this is the amide; ^-NH₂ is a very poor leaving group, but if is protonated it is much better.
In terms of general reactivity towards nucleophiles, the trend is with how good the leaving group is;

Acid Chloride > Anhydride > Ester ~ Carboxylic Acid > Amide

At the right hand end of the scale, the carbonyls are made less reactive by resonance donation from their X group into the carbonyl π system - shown here for the amide;

This delocalised π system holds the amide in a planar conformation - another way to put this is to say there is partial double bond character between the carbon and the nitrogen (although the majority of the double bond is still between the carbon and the oxygen). The effect of π donation into the C=O system is to lessen the polarity of the bond and make it less attractive to incoming nucleophiles.