What are the important points about cryptographic hash functions?

Question

I was reading this question on MD5 hash values and the accepted answer confuses me. One of the main properties, as I understand it, of a cryptopgraphic hash function is tha

Answer 1

"why aren't MD5 hash values reversible?" is because "an infinite number of input strings >will generate the same output"

this is the reason that it isn't possible to reverse the hash function (get the same input). cryptographic hash functions are collision resistant, that means that it's also hard to find another input value that maps to the same output (if your hash function was mod 2 : 134 mod 2 = 0; now you can't get the 134 back from the result, but we can stil find number 2 with the same output value (134 and 2 collide)).

When the input is smaller than the block size, padding is used to fit it to the block size.