Email forwarding bounces your email to another address that you provide. Instead of your mail going into your Yahoo! Inbox, it's forwarded to the other account's inbox so you can check it there. While forwarding is "on," you won't get any mail in your Yahoo! account.
POP is an acronym for the way messages are sent from servers to the email application—or client—that you use. POP access means you can get access to one of your email accounts (your account at school, for example) from another email account (your Yahoo! account). With POP access, you can send and receive messages in different accounts all from one place.
With Yahoo! Mail you can: