Yahoo! does not provide a standard way to access your email using the IMAP protocol.
While they have a publicly available IMAP server at imap.mail.yahoo.com, it uses a non-standard version of the IMAP protocol. Normal email clients like Microsoft Outlook will not be able to connect to it on their own.
However, you can download a helper application called Yahoo IMAP Connector, which will make the necessary translations between your email client (e.g. Microsoft Outlook), and the Yahoo IMAP servers (imap.mail.yahoo.com) to make everything work correctly.
First, you will need to download Yahoo IMAP Connector. It works completely free for a 15 day trial, after which you will need to purchase it for $9.95.
Once you have downloaded and installed Yahoo IMAP Connector, you can use the following settings to get full IMAP access to your Yahoo! email:Incoming Server
Yahoo! also provides POP3 access through their Yahoo! Mail Plus program for $19.99 per year, but POP3 is an awkward way to manage your email from your desktop.
POP3 is only meant to be used when you have one email client, like your desktop computer, and you want to periodically download the email you receive to your desktop. This means that once the email is downloaded, you can't access it from the regular Yahoo! web interface. Only one copy of each email is typically kept, and that is on your desktop computer.
So, for example, if you receive your email via POP3, the emails are downloaded and removed from the Yahoo! servers, and stored on your desktop computer.
If you have already signed up for Yahoo! Mail Plus, here are the settings for receiving your Yahoo! email through POP3:Incoming Server