Interacting, or colliding galaxies, result from the gravity from one galaxy disturbing another. These interactions range from minor, where a distant satellite galaxy disturbs the spiral arms of a primary galaxy, through to major, with a full blown galactic collision.
Many years of research indicate that galaxy collisions are fairly common in galaxy evolution. However, due to the vast distances between stars within a galaxy, these are not collisions in the normal sense of the word. Rather, they take the form of gravitational interactions as galaxies pass by, or even through, each other.
Major interactions can lead to mergers, where colliding galaxies do not have enough momentum to escape following a collision. The galaxies are subsequently drawn back towards each other, and after several passes will merge to form one galaxy.