For the surfers, an IP is like a telephone number for your computer connection. When you get online, you are assigned an IP address(much like a telephone number if it makes it easier to comprehend) when you log in. Whether from dial up, or a broadband connection.
A static IP means your number will always remain the same. So no matter when you are on, you will always have the same IP number. A dynamic, like broadband used mainly (although you can have a static) will change. No, then whole number doesn't change, but normally the last sequence will. So it's harder to pin you down.
With broadband, depending on ISP, this change change per log in, or session, or can randomly change through the course of a time period. There's no hard fast rule.
Back on topic, again what you can do with an IP can vary by ISP. Some will provide you more detail than others. Mainly an IP's biggest threat comes from hackers, and people who like to play the ping game, boot you from chats, try and gain access to your system, and so forth. Which is why have good firewalls, both software, and hardware is key.
