A neutron star is an extremely dense object that forms after a core collapse of a massive star during a Type II, Type Ib or Type Ic supernova. It mainly consists of neutrons with relatively few electrons and protons. It can have a mass of between about 1.4 and 3 solar masses and has a diameter of about 20 km. Neutron stars have densities of below 1 x 10⁹ kg/m³ in the upper layers or crust up to about 8 x 10¹⁷ kg/m³ deeper inside. That's more or less the density of all 7 billion human beings packed into the size of a sugar cube. Surface gravity is 200 000 000 000 times stronger that the surface gravity here on Earth, so anything that has the plan to escape from a neutron star should be prepared to accelerate to a third of the speed of light, otherwise it's going to drop back again onto the neutron star. If you fell from a height of 1 metre towards a neutron star you'd hit it in less than 0.000001 seconds with a speed of 7.2 million km per hour. Ok, enough numbers, it's a weird place.

The crust of the neutron star consists of a solid lattice of nuclei with electrons flowing around them. The deeper we enter, the more massive the nuclei get. At a certain depth we reach the point of the so-called neutron drip. From now on the neutrons start leaking out of the nuclei so that we have a "soup" of nuclei, free neutrons and electrons. When we come down to the core of the neutron star it's not certain what the exact state of matter within it is. One model describes it as superfluid neutron-degenerate matter that is composed mainly of neutrons with some protons and electrons. There are concurring alternatives like degenerate strange matter models (with "strange quarks") and other models. The nature of the centre of a neutron star is still outside the certain knowledge of science at this time.

You can read more about the formation of neutron stars in our supernova article. If you want to better understand the strange state of matter called degeneracy you can read the corresponding article on Sun.org.