What governs the trajectories of galaxies?

I learned from TV that space is expanding and galaxies are getting farther and farther away from each other. Yet some are getting also closer and even going to collide! for ex. Andromeda and the Milky Way. So do the galaxies exercise a pull on each other? and what is the form of their trajectories? do they travel in a straight line, in a curve, around what? to where?