What are unipotent cells?

All cells arise from pre-existing cells, and as an organism develops from the fertilized egg (zygote), cells undergo specialization and 'differentiate' into some particular cell type based on temporal and spatial cues.

At first, cells of the zygote divide to form a mass of cells that are 'pluripotent', in that they can become any cell type. Afterwards, around the stage of gastrulation, cells are 'multipotent' in that they are already committed to forming only cells of the ectoderm, mesoderm or endoderm.

With time and further specialization, cells become progressively 'lineage restricted', and they reach a state where they become precursors of only one particular cell type.

So what makes unipotent cells different from terminally differentiated cells?

Unipotent cells can proliferate, self renew and divide asymmetrically (produce one copy of itself, and one differentiated cell). They are also not yet functionally active as the cell type that they are destined to become. They usually remain quiescent as a 'side population' in a tissue until the requirement arises for them to mobilize, proliferate or differentiate.

What is their purpose?

They can serve as a reserve of cells that are ready to generate large numbers of the cell type that they are restricted to.

One such example is that of Skin stem cells. They are present underlying the epidermis, and are poised to proliferate extensively and form new skin in the event of an injury. Unipotent stem cells are also found in almost any tissue which has to regenerate on a regular basis.