The first part of Chapter 12 discusses the reactions of Class I and Class II carbonyl compounds with strong (C and H) nucleophiles. Starting the chapter by comparing the reactions of the two classes of carbonyl compounds with the same nucleophiles lets you appreciate the difference in the way the two classes of carbonyl compounds react. Because Class II carbonyl compounds (aldehydes and ketones) do not have a group that can be replaced by another group, they undergo a nucleophilic addition reaction with one equivalent of the nucleophile. In contrast, Class I carbonyl compounds undergo a nucleophilic acyl substitution reaction, reacting with two equivalents of the nucleophile. The first equivalent replaces the leaving group, forming a Class II carbonyl compound, which then undergoes the normal addition reaction of a Class II carbonyl compound with the second equivalent of the nucleophile. Be sure to notice that what you learned about the partitioning of a tetrahedral intermediate (it preferentially eliminates the weaker base) in Chapter 11 applies to many of the reactions in this chapter as well.
The chapter goes on to discuss the reactions of Class II nucleophiles with poorer (O and N) nucleophiles. Here you will see that because the tetrahedral intermediate is bonded to two electronegative atoms, it is unstable and eliminates water when it collapses.
The last part of this chapter deals with the reactions of α,β-unsaturated carbonyl compounds. These compounds have two sites that can be attacked by a nucleophile. The particular site that is attacked depends on what nucleophile is used, the structure of the carbonyl compound, and the conditions under which the reaction is carried out.