There are two ways an ISP can allocate or assign an IP address to you for use on the internet: static or dynamic.
Static IP Address
A static IP address is assigned to your computer by your ISP and is permanent. Each time you connect to the internet, the same address is used. This address is never used by any other computer or device.
Dynamic IP Address
It would be simple if every computer that connects to the Internet could have its own static IP address, but when the Internet was first conceived, the architects did not foresee the need for an unlimited number of IP addresses. Consequently, there are not enough IP addresses to go around. To get around that problem, many Internet service providers economize on the IP addresses they possess by temporarily assigning an IP address to each computer from a pool of IP addresses. The temporary IP address is called a Dynamic IP address. As a result, the IP address may be different each time the computer connects to the Internet.