I'll explain it to the best of my knowledge:
Dynamic IPs are generated once every few hours typically, sometimes days. The reason for them is because there are more internet devices (modems) than there are IP addresses using the current format. So if everyone had a static IP, there wouldn't be enough of them to go around. Therefore dynamic IPs were created and are assigned to computers when they access the internet. They typically last for a little while, and when it expires, your modem requests a new IP and is granted a different one.
On the other hand, static IPs are permanently assigned numbers that are useful for servers because you can assign DNS names (names like, yahoo.com or espn.com) on static IPs only. Yahoo actually has a static IP you can access their site through: http://126.96.36.199
So if you're not hosting a server, stick with the dynamic IPs, because they are usually a LOT less expensive.