A teaspoon of matter from one of these star cores weighs one hundred million tonnes on earth. These are formed at the end of a stars life. If the core mass can't support itself, then the neutron star's degenerative pressure collapses it into a singularity, popularly known as a Black hole.

Since the majority are formed from the collapsed core of a dead star, the gravitational pull they exert is about the same as any large star; you could even orbit around one safely and there might even be a few rocky, deep fried planets still floating around one. The real danger is the lethal radiation they emit, which is in the form of intense Gamma and X-ray bursts; these are variable in intensity, but can be huge if the Black hole is in the process of swallowing anything big (such as a planet or the outer atmosphere of a binary star), so caution is strongly advised in approaching any Black holes.

Spinning black holes (known as Kerr singularities) are even more unusual in that the singularity at their centre is spinning so fast it forms a ring; through which it may be possible to travel through space and time (nobody knows for sure). That said, nothing can survive a descent into any black hole, the tidal forces are so strong that any object is reduced to a stream of elementary particles, and the time distortion is so intense that (from the plunging observers point of view) the Universe seems to speed up until it winks out of existence.