Neutron stars are compact objects that are created in the cores of massive stars during supernova explosions. The core of the star collapses, and crushes together every proton with a corresponding electron turning each electron-proton pair into a neutron. The neutrons, however, can often stop the collapse and remain as a neutron star.
Neutron stars are fascinating objects because they are the most dense objects known. They are only about 10 miles in diameter, yet they are more massive than the Sun. One sugar cube of neutron star material weighs about 100 million tons, which is about as much as a mountain.
Like their less massive counterparts, white dwarfs, the heavier a neutron star gets the smaller it gets. Imagine if a 10 pound bag of flour was smaller than a 5 pound bag!
Neutron stars can be observed occasionally, as with Puppis A above, as an extremely small and hot star within a supernova remnant. However, they are more likely to be seen when they are a pulsar or part of an X-ray binary.