Question on additional IDs and sub-keys

[Luca Manini]

Hi, 

I've just started "learning" pgp, so please be patient. 

I've managed to create my key, exchange it with a friend, 
send and receive messages and so on. So the basic is ok and may be I
do not need nothing more, but while studying...

So I don't understand additional IDs and sub-keys. 

It seems to me that additional ID's are just different "additional
info" (name, email address). If I encrypt a message for two different
ID's of the same KEY, what's the difference?

Subkeys are also obscure. When I encrypt a message can I choose the
subkeys to be used?

	Any help is welcome.
	Bye, Luca.