IP-address (short for Internet Protocol Address) - a unique identifier (address) of the device (usually a computer) connected to the Internet.
Each device (laptop, computer, mobile phone, tablet, game console, TV, dedicated server, etc.) on the Internet has its own IP-address. Since you are currently connected to the Internet - it means that your Internet device also has a unique address on the network. However, you can be connected to the Internet through a router on your local network. In this case, your Internet device seen from the Internet with the address, which is your router.
IP-addresses v.4 consist of four numbers (from 0 to 255) separated by dots and look like 192.168.0.100 or 126.96.36.199.
IP-addresses v.6 consist of eight groups of four hexadecimal digits separated by colons and look like 2001:0db7:10ab:08c7:14f4:8a2e:02a0:663d or 2001:DB8::/32
Since these numbers are usually assigned by the ISPs in the region on the basis of blocks, IP-address can be used to determine the region or the country from which the computer connects to the Internet.
As for the man to remember the IP-addresses cause quite tiring, there are special database matching the IP addresses symbolic names, which are easier to remember. Such names are called nodes (hostname). Nodes can be converted into IP addresses and vice versa.
The IP address can be static (in the event that an individual user provider assigned a permanent address), as well as dynamic (if the provider gives the user an IP address when connecting from a pool of available addresses by DHCP).
Also one computer-based virtual hosts can act as multiple devices with multiple IP addresses and hosts. For example hosting services on the Internet.