IP-address (Internet Protocol Address) — is a unique numeric identifier (address) of a device (usually computer), connected to the Internet.

Each device (computer, laptop, dedicated server, mobile phone, etc.) on the Internet has its own IP-address. Since you are currently connected to the Internet - it means that your computer also has its own unique address on the network. But you can be connected to the Internet through a router or gateway in your local network. In this case, your computer from the Internet visible with the same IP address, which has your router or gateway.

IP addresses can be static (when provider assigned a permanent address to the user), and dynamic (if the provider gives you the IP address from the pool of free addresses on DHCP when you are connecting). Static IP-address provides possibility to organize permanent access to the same computer through the time. Vice versa, when is used dynamic IP there is no guarantee that using the same address the same device will be accessed. As far as personal computers usually do not require permanent access, providers assign to such users dynamic IPs. Dynamic IP assignement gives a way to save address space for providers and gives them an ability to serve more clients with the less address space. That's why obtaining static IP from provider costs some additional money to the regular paymants for Internet access.

In addition, built on the basis of virtual hosts computer can act as multiple devices with the multiple IP addresses and sites. For example, web hosting services on the Internet. So, as you can see there could be situations when to one computer multiple IP-addresses are assigned. Usually in such cases it is assignements of several static IPs, but in some situations it is could be a mix of static IPs and dynamic IPs on a different network interfaces of the computer.

Knowledge of your IP address allows to provide an access to services and programs on the computer (games, chat rooms, FTP, remote access to desktop, etc.)

For today there are two different types of IP-addresses used (in the terms of technical protocols) - IPv4 and IPv6.