Intro to Enolates

The α-protons of aldehydes, ketones, esters and amides are relatively acidic. 

They can be deprotonated by a number of different bases to different extents.  The choice of the base is crucial to predicting reaction outcomes.  The conjugate base resulting from the loss of an α-proton in an aldehyde, ketone, ester or amide is an enolate.

Deprotonation with weak base (^-OH, ^-OR)

If we rewrite the above equilibrium using hydroxide ions we obtain the following.  Recall that the equilibrium will proceed to the side of the weaker acid (and weaker more stable base).  In this case, the equilibrium is on the reactant's (left) side.  There will be a small amount of enolate in equilibrium with the aldehyde/ketone.  These are good conditions if you want the enolate to react with the aldehyde as in an Aldol reaction we will see soon.  Because we only get a small amount of enolate and most of the aldehyde remains we say that it is non-quantitative deprotonation.

Deprotonation with strong base (^-R)

If you were to attempt to use a very powerful base such as butyl lithium, while the base is sufficiently strong enough to deprotonate the aldehyde, there would also be a significant addition to the aldehyde as well.  This is not good as it takes away from your yield.

Deprotonation with strong hindered base (LDA)

Lithium diisopropyl amide (LDA) is one of many bases purposed for forming enolates quantitatively.  The pKa of the conjugate acid of LDA is 36, so it is powerful enough to convert all of the aldehyde to enolate.  Because of the large isopropyl groups, the addition of the amide anion to the aldehyde is limited because of sterics.