Latex-Modified Mortar
This type of mortar is the most common among the mortars available and contains latex. Latex makes the mortar more sticky and elastic. You can use this type of mortar in many installations, either in external or internal locations. However, latex-modified mortar takes a longer time to dry than other mortars.
Unmodified Mortar
Unmodified mortar does not contain polymer or latex additives. It is considered the lowest quality of all mortars available and is among the cheapest. As it does not contain latex, it is less sticky and is not suitable to apply on wood surfaces. Unmodified mortar is not used very often in installations, as the installation areas move over time when this mortar is applied. It is suitable to apply on exterior applications where you install the slate wall tiles directly onto concrete surfaces, such as chimneys.
Source: www.DoItYourself.com