It's straight forward enough .... ish :)
When the CCTV system is set up, it's connected to the internet (usually by using an Ethernet cable, but it can be connected wirelessly). It is then set up, following the manufacturer's instructions, so that the system has a user name and a password.
An app is downloaded onto your mobile, or other mobile device, so that when the app is opened, you can connect to the CCTV system by logging in using your unique user name and password.
It doesn't matter where you are - you'll be able to view your CCTV using your mobile, as long as the CCTV system is connected to the internet and your phone is connected to the internet. It doesn't matter how they're connected, as long as they're both connected to the internet.
Here's a system very similar to the one that I've got:
http://www.ebay.co.uk/itm/1TB-CCTV-8CH-AHD-HDMI-DVR-1500TVL-Outdoor-Camera-Security-Surveillance-System-/331761123459?hash=item4d3e810083:g:insAAOSwGotWndUW
If you scroll about 3/4 down you'll see:
"Support Mobile IPhone, IPad, Android, Table PC"
This is a list of the OS (operating systems) that it's compatible with ..... any more questions, feel free to ask :)