What is the difference between transaldolase and transketolase in the pentose phosphate pathway?
An aldol is a $\beta$-hydroxy ketone or aldehyde, and is the product of an aldol addition.

The enzyme that cleaves the bond formed (red) is known as an aldolase. The transaldolase reaction begins by aldol cleavage that is, cleavage of the bond in red.

Transketolase on the other hand simply cleaves off the ketone group from the substrate. It does not perform an aldol cleavage.

Apart form this, the mechanisms of action of the two enzymes vary considerably. For instance transketolase requires thiamine pyrophosphate (TPP) as a cofactor but transaldolase does not.