Email is just connected to a server somewhere (i.e., in your case, Virgin's systems).
You can access this from anywhere you get an internet connection. But there are two ways of accessing email right now (popular ways, anyway):
POP: this means that emails are fetched from Virgin's systems onto your computer, and often taken off Virgin's machines.
IMAP: this means that emails are kept on Virgin's systems, and you just use your computer to read them on there.
IMAP has multiple advantages, especially if you're using multiple machines to access your email. With POP, you could check your email from computer 1, so that the email goes on there. But then, checking from computer 2 the next day, those other emails are stuck on computer 1 -- not too helpful.
If you use IMAP, then you can check from computer 1, then go and check from computer 2, and everything stays in sync, because they're just reading from the same Virgin system. This is why Chuck recommends IMAP.
When you set up Outlook, you tell it to use IMAP or POP. Check your settings to see what you have set. In Outlook's account options, your incoming mail server will be set to something like pop.virgin.co.uk, or imap.virgin.co.uk, etc.