Upgrading from 4.6 to ECC 5.0 and Unicode related query

Hii

My client has SAP 4.6C version and wants to upgrade it to ECC 5.0. As an ABAPer, I am asked to know and understand what is to be done Technically in this upgradation. What advantages/features ECC 5.0 has over 4.6C. I have read somewhere that Unicode programs need to be taken careof. Can anyone send me some detail notes on what Unicode means and unicode programs differ from non-unicode ones. How is unicode programs taken careof during upgradation.

Thanking you all,

Abdul