Aldehydes are organic compounds characterized by the presence of a carbonyl group ($C=O$) bonded to at least one hydrogen atom. They have the general formula $RCHO$, where $R$ can be a hydrogen atom or any alkyl or aryl group.
Ketone: An organic compound with a carbonyl group ($C=O$) bonded to two alkyl or aryl groups. They have the general formula $RCOR'$.
Carboxylic Acid: Organic compounds containing a carboxyl group ($-COOH$). They can be formed by oxidizing aldehydes.
Ester: Derived from carboxylic acids and alcohols, esters have the functional group $RCOOR'$ and are known for their pleasant fragrances.