Study of dynamics of action of unipotent subgroups on homogeneous spaces and its applications to Diophantine approximations has been attracting considerable attention over the past 40 years or so. Margulis's celebrated proof of Oppenheim conjecture and Ratner's seminal work on Raghunathan's conjecture are two inspiring works in the subject. Although Raghunathan's conjectures in characteristic zero have been settled affirmatively, very little is known in positive characteristic case. In this talk we will address this issue. In particular the main focus will be on a recent joint work with M. Einsiedler on classification of joinings for the action of certain unipotent subgroups. As it turns out this has some applications to quasi-isometries of lattices. This connection is drawn explicit by K. Wortman in an appendix to our work.