I have been told of the Milky Way galaxy but never seen it. What is it?

Stars are formed into major groupings called "galaxies." Our galaxy, the Milky Way, is visible as a fairly bright band of light across the sky in the summer. The Romans thought it resembled a road paved with milk, so that it why it is called a "way." You have probably seen it but never realized it. It looks like a diffuse jet contrail or a band of clouds. It helps to be out of town.

How many stars are in a galaxy?

It varies, but our galaxy is a middling to large one at 100 to 200 billion stars. That is what is lit up, however. The mass, much of it dark, is even greater.

How much of the galaxy can we see?

We can see most of it to toward the outside, but looking inward to the core, we really can't see much farther than the distance light travels in 8000 years, called 8000 light-years. We can't see the center of our own galaxy, though we can detect it in more penetrating radio wavelengths.

What happens when galaxies collide?

Not much. There is some radio emission from dust and gas clouds hitting each other at high speed, and some extra star formation, but stars occupy so little of the volume of space that real collisions would be few. The galaxies mostly float through one another, except for the bigger galaxy stealing stars from the smaller galaxy (some things never change). Of course, galaxies collide very slowly, but there are enough cases that we can see the process at all stages. The interesting effects are the changes in the way galaxies look after the collision. Ring galaxies always are in the presence of another galaxy, so it is thought that the ring is an aftereffect of a central collision. One thing that could happen, though, is that enough mass would be thrown at a quiet supermassive black hole at its core that it would trigger some activity like x-ray or radio emissions.