A bead of water on a window is a tiny curved mirror and a tiny curved lens, and the street behind it appears inside the bead, upside down and compressed. The big drops run, and as they run they pick up the small ones and leave a trail of them behind.
Surface tension holds a drop in a sphere. On a vertical pane of glass the drop is squashed against the surface, and it sticks because of the wetting of the glass — water climbs a short way up the inside of the drop and is held there. Gravity pulls the drop down. The two balance until the drop is big enough that the weight beats the adhesion.
Then it starts to run, and as it goes it collects the smaller drops it meets, which is why a running drop on a window accelerates as it descends and why the trail behind it is a row of stationary beads. The same physics gives you the fat drops that race down a car window in winter.