Enolates are anions at the the alpha position of carbonyl compounds like ketones and aldehydes. They can participate in many kinds of reactions including tautomerization, alkylation, acylation, condensation, etc. 

Enolate Formation

Enolate-formation-oxyanion-and-carbanion-resonance-structure-contributorsEnolate formation, oxyanion and carbanion resonance structure contributorsEnolate anions are intermediates of base-catalyzed tautomerization (aka tautomerism). Carbonyls and enols (aka alkene-ols) are tautomers and exist in equilibrium 

Tautomerization-mechanismTautomerization mechanismReactions:

Alkylation and Acylation

Enolate-alkylation-and-acylationEnolate alkylation and acylation Enolates can undergo alkylation and acylation reactions. In these, the carbanion acts as a nucleophile to attack an electrophile like an alkyl halide or acyl chloride. Acylation reactions form beta-dicarbonyls

Condensation Reactions

Enolates can also attack carbonyls directly through nucleophilic addition, and in this process larger molecules are formed. Condensation reactions like Aldol, Claisen, and Dieckmann

Claisen-condensation-mechanismClaisen condensation mechanism