It can be helpful to see that most of the reactions of aldehydes and ketones fit into either one of two categories. For one group the mechanism relies on the partial positive charge of the carbonyl carbon, which makes it subject to attack by nucleophiles. Examples include acetal formation and imine formation. In the other group the reaction depends upon the nucleophilicity of the enolate α carbon. In other words, most reactions involving aldehydes or ketones are either involving nucleophilic attack upon the carbonyl carbon or they involve keto-enol tautomerism with electrophilic attack upon the α carbon. Aldol addition is a reaction that can't be said to belong solely to either category because it belongs to both. In aldol addition the enolate form of one aldehyde or ketone acts as a nucleophile to form a bond with the carbonyl carbon of another aldehyde or ketone (or another equivalent of the same aldehyde or ketone.)