A IP address is a designated address given to an internet connection. Much like your home's street address an IP address designates your location on the internet. An example of an IP address would be 192.168.0.1
An IP address can appear to be shared by multiple client devices either because they are part of a shared hosting web server environment or because a proxy server acts as an intermediary agent on behalf of its customers. In which case the real originating IP addresses might be hidden from the server receiving a request. The analogy is an apartment complex where the main street address is shared amongst several residents.
IP addresses are managed and created by the Internet Assigned Numbers Authority. An IP address is also assigned to your computer when you connect, or setup a connection, to the internet through your ISP.