I recently set up a Gmail account for a business and because they are running multiple computers I set them up with IMAP settings for their G Mail account.
The advantage of using IMAP is that you can check your mail from multiple computers and your email will always be in sync.
Your messages will stay on the server until you decide whether you will download them or not to your local hard drive.
However if you delete a message from one computer you will delete them from all computers.
Your mail accounts are now designed to be " mirror images " across all your computers.
Here is the Google explanation.
A two-way communication path (IMAP). Unlike with POP, your devices talk back to our servers and sync your changes automatically with IMAP. When you sign in to your Gmail account in a web browser, actions you've taken on your email client or mobile device (like putting a message in a 'work' folder) will also appear in Gmail (your message will already have a 'work' label on it). This all happens automatically once you set up IMAP, so you don't have to read or sort all your mail twice. This is really helpful when accessing Gmail from multiple devices.
point , well after setting up the account and explaining the above they
went into one computer deleted all their messages as they thought they
would have them on the other machines.