As we speak, there are a few hundred pairs of stars in collision somewhere in the universe. A few different things can happen when stars collide. If the stars are similar in size and mass and approach each other at low speed, they will probably merge into a bigger, more massive star. If a smaller star, like a white dwarf or neutron star, smashes into a bigger star, like a red giant, most of the giant's outer envelope would be blown off as it absorbs the impact. The results get a little more violent when two smaller stars collide.
Neutron stars are very small and dense.
If a neutron star reaches a certain mass, it will implode and form a black hole. Therefore, if two neutron stars merge but their combined mass is more than the maximum mass a single neutron star can have, then it implodes into a black hole. If the circumstances are the same when two white dwarf stars collide, it will implode into a neutron star. In this case, the stars collide and begin to merge together. And we see gamma ray bursts from distant regions of the universe, so they may be the product of stellar mergers or collisions.