Unicode is an umbrella term to describe the family of standards that are designed around the notion that a single, unique number should be assigned to every character, of every alphabet, in every language in the world.

It also assigns unique numbers for many writing functions, as well as punctuation marks.

UCS-2 is a 16-bit code-space, while UCS-4 is a 32 bit code space. Most of the characters that are numbered in Unicode fit in the first 64K (16-bit) code points, but not all. UCS-4 (32-bits) provides enough code-points for all the characters currently numbered in Unicode, and many future characters.