You might want to read this post* first if you're not sure what a supernova is.
In short, a supernova is an exploding star. What we see from Earth, the supernova remnant (SNR), is an expanding cloud of glowing matter that cools gradually. The picture to the left was taken by the Hubble Space Telescope and depicts what is known as the Crab Nebula. It is the remnant of a Type II supernova in the constellation of Taurus, and is arguably the most well known supernova remnant in popular astronomy.
The supernova remnant is what we see after a star explodes, but...
What is left behind?
Type I supernovae:
- Type I supernovae typically don't leave anything behind at all: all of the star's matter, including its iron core, is blasted into space.
Type II supernovae:
Type II supernovae usually leave behind one of three objects:
- A neutron star
- A pulsar (this is just a spinning neutron star, really)
- A black hole
Which is formed depends on the original mass of the star and, more importantly, the mass that's left over after the supernova.