In essence it is a point of zero size, but infinite mass - a star that is so large that when it has spent all of it's nuclear fuel it begins to implode in onitself. However as it's mass is already so large the more it implodes in on itself, the greater it's mass becomes, so that eventually you end up with a run away process where as the star collapes or shrinks it becomes more and more dense, and the more compacted it becomes, the greater the gravitational force it generates - until eventually the start becomes so compacted it collapses to a point so small it essentally has no size whatsoever - but which (as stated) still has infinate mass. For this catastrophic collapse to occur, you need a star that has a mass that is at least 3 1/4 times the size of our own sun.
However it still obays Newton's theory of Universal Gravitation (and later Einstein refinements also), which basically state that 'every object in the Universe attracts every other object with a force directed along the line of centers for the two objects that is proportional to the product of their masses and inversely proportional to the square of the speration between the two objects.'
http://csep10.phys.utk.edu/astr161/lect/history/gravity.gif
Or in other words, the further away you are from (any) body of mass in the Universe, the weaker its gravitational influence becomes. This is true also of black holes - even though at the exact point the black hole exists, gravity itself can be considered to be infinite.
It also makes no sense to say 'what would happen if you entered a black hole'. Whether string theory is correct or not (in that information might be able to escape a black hole) what is clear is that at the exact point that a black hole exits all matter is stretched so thinly so as to become exactly infinately thin. We are not just talking about spaghetti here - we are talking about the thinest spaghetti it is possible for you to imagine, indeed as I said infinately thin. Asking what would happen if I entered a black hole, is a bit like asking 'what would happen if I threw myself into a volcano?' The answer is that you would die - and anything that went in there with you would be destroyed. The only difference being that a black hole is an infinite number of times (again literally) more deadly than any volcano ever could be.
It also makes no sense to ask what happens 'inside a black hole'. There is no such thing as 'inside', you can't have an inside of a point of infinite mass and infinite density and zero size. The term 'inside' literally has no meaning. Nor does time, or motion, or or up or down, or big or small any of these things. The point at which a black hole exists (which is known as a singularity) is a point at which the known laws of physics stop working - which some might argue is Hawking mistake, because he is using known physical laws to describe an object where no such laws exist.
BTW for anyone moving towards a black hole (given that this is not advisabe) their time would not slow down to a crawl and then eventually stop as some here have described. Or at least not from their perspective. For an observer moving towards a black hole their time (to them) would continue to appear to pass normally. However they would notice everyone else in the Universe would appear to be moving progressively more slowly. However, from our perspective it is our time that would seem to move normally - while it is the person moving towards the black hole who's time would seem to pass more slowly. Time is not the fixed thing you might imagine it to be - it is as Einstein described it, 'relative.' It depends on where you are in the Universe much more than 'when' you are.
I trust this clarifies the subject discussed here.
A bunch of black holes, each with infinite mass, would sure solve the 'ol dark matter problem and there would be a "big crunch" in the near future...
