Enol

An enol is an alkene that has an OH group attached to one end of the double bond. It is called an enolate if the proton on the oxygen atom is taken away.
Enols are tautomers of ketones or aldehydes. Tautomers are molecules which are different only in the position of an hydrogen atom. Enols can be made very easily from ketones or aldehydes using a base. They are quite unstable, because the C=O bond is stronger than the C=C bond. However, they can do many interesting reactions. The best example is the aldol reaction.
Enols of ketones can be made on two different sides. It is important to be able to choose which side to react. The side with less substituents is the easiest one to react: it can be made at low temperature with a bulky base. This is called the kinetic enolate. The other side, with more substituents, is more difficult to react. Higher temperatures are needed. This is called the thermodynamic enolate.