Conjugation in bacteria
Within a bacterial cell exists circular chromosomes, and plasmids. During cellular division, chromosomes are always conserved - passed to both daughter cells. 

As you are aware, plasmids can be passed between cells by a form of horizontal gene transfer called conjugation - whereby plasmid DNA is replicated, and passed through a sex-pilli into an adjacent cell.

When a cell divides in to two daughter cells, it is possible that one cell will not receive a plasmid. Thus, the result is one F+ daughter cell, and one F- daughter cell.