Two molecules of an aldehyde or ketone containing α- hydrogen atom, combine itself in the presence of dilute alkali to form β-hydroxyaldehyde or β-hydroxyketone. 

This reaction is called aldol condensation after the name of the product which contains both aldehyde (al) and a hydroxy (ol) group.

Similar condensation of two molecules of ketones ( α- hydrogen) is also considered as aldol condensation although the product in this case contains a keto and hydroxy group rather than an aldehyde group.

What happens to aldol on dehydration?

They form α,β-unsaturated carbonyl compounds