Question

What is the difference between an unconditionally secure cipher and a computationally secure cipher?

Answer 1

A secure cipher is known as an “Unconditionally secure cipher” if the cipher text that is generated by it does not information which determines the corresponding plain text uniquely, irrespective of how much cipher text is available.

  

A "Computationally secure cipher" is a kind of encryption scheme, in which the cost of breaking the secure cipher exceeds the value of the information that is encrypted and also the time that is needed to break the cipher is more than the lifetime of the information that is useful.