What happens after two galaxies collide?
The Milky Way Galaxy and the Andromeda Galaxy are in fact on a collision course. But even so, it is unlikely that there will be a single collision between any two stars of the colliding galaxies. This is because of the great distances between any two stars.
This picture above is the ongoing collision of galaxies NGC 5426 and NGC 5427. These spiral galaxies resemble our galaxy and the Andromeda galaxy. As can be seen, the gravity of the galaxies is "stealing" material from the other galaxy as they begin to collide.
It is impossible to predict what our galaxy will look like after it collides with Andromeda, but many speculate that it will be a much larger version of what we see today.