UTF-8 Unicode encoding and country specific characters

UTF-8 Unicode encoding and country specific characters

I'm a bit lost concerning encoding. I do not understand why we say "UTF-8
Unicode". "Unicode" sounds to me like all possible characters in the
world, which doesn't fit in a single byte char.
Can you explain this to me please ?
A second question : if i decide to use single byte chars with that "UTF-8
Unicode" encoding in a program, will i be able to handle most european
characters ? What about russian, arab, chinese, etc ?
Thanks for your help.