You may be asked whether you want a fixed or dynamic IP address

In simple terms, a fixed IP address means the IP address which identifies your computer on the internet stays the same all the time.

This is important if you are running a small business and want to run a server which can be accessed from outside (other computers need to know the address of your server to be able to talk to it).

Otherwise a 'dynamic IP address' means a different address gets allocated automatically every time you connect to the internet.

This can be cheaper for ISP's to provide and for most home users it's fine.

SOURCE  http://www.bbc.co.uk