How do we communicate over the internet?
With POP3, you retrieve Emails from the server and only the ones you download will be stored on your device. The advantage of this is that it doesn't use up your data allowance and doesn't fill up the storage if you are using a mobile phone.
With IMAP4, Emails are pushed onto your device. No matter how much your data allowance is, it will also fill up your storage. The advantage of this is that you can retrieve important Emails instantly.
Skype and Facetime are examples of VoIP with video. If the video is turned off, this would be VoIP. An advantage of VoIP is that it is free (provided your internet connection is free), so you save on phone bills.