Understanding of the difference between a stream cipher and a block cipher

Explain the following concepts:

What is your understanding of the difference between a stream cipher and a block cipher?
What are two ways to launch a cryptanalytic attack against a cipher (based upon what the attacker knows or has in possession)? Be sure to provide detailed examples with supporting research.
What is your understanding of the difference between an unconditionally secure cipher and a computationally secure cipher?
Your organization has asked for a cryptographic algorithm recommendation for a new project that requires high-level security. What algorithm would you recommend and why?