POP3 vs IMAP
POP3 and IMAP are two different ways of checking mail. A mail client program connects to the mail server using either POP3 or IMAP. All DreamHost mail accounts support both POP3 and IMAP connections automatically.
POP3 downloads all mail from the server from the inbox and stores it on your computer. The emails are removed from the server and only stored locally in your mail client program. Emails are available when you're not connected to the internet.
IMAP syncs your mail client program with the server. Emails stay on the server, and you can make and view mail folders on the server in addition to the inbox. Most mail client program have a feature to initially sync just the email headers, so you can quickly see what emails you have, then download the message body when you want to read the email. Since emails stay on the server, you can see all your emails from many mail client programs or devices. Webmail uses IMAP.
Which should you choose?
Use IMAP if you want to check email from multiple computers or devices. Use POP3 if you want your emails accessible always, even when there's no internet connection.
If you have been using IMAP and have some mail stored in folders other than the inbox, move the emails to the inbox before using POP3.