IMAP and POP are the protocols that are used when downloading email from a mail server, and most email clients (such as Outlook, Thunderbird, Windows Mail) will accept either when configuring email accounts. VentraIP supports the use of either.
What you ultimately decide to use is completely up to you. One is better than the other in certain situations.
All your email will be downloaded to the device you access it from (into the mail client) and deleted from the mail server. This will allow you to read your emails while online or offline.
However this also means it will be using your disk space on the device and if it ever has an issue any emails stored on it may be lost.
When you read your email the mail software will store the email on the mail server and make a copy to your device you are connecting from. The email remains on the server, and you can access it via another IMAP mail client and view your mail exactly as it appears in your other devices, which will keep everything in sync.
Most email clients will cache IMAP email, however with IMAP email you cannot manipulate your mail whilst offline (not connected to the internet) as it requires an active connection to the server. Furthermore you don’t retain your own local copy so if you delete your email account, all the email is deleted as well.