A randomly assigned internet protocol from a pool of IPs owned by your ISP. The IPs are all usually in the same range, and you are given the first free IP when you log on. This means that your IP will change every time you log on (although there is a chance that you could dial up an IP used before).
The benefit of this method (as opposed to static IP is not security, but rather the ISP not having to maintain an IP for every customer, thus saving money. However, this means that only a set number of customers can be online at a time. This also makes the connection slower.
AOL and its subsidiaries (compuserve etc.) use a dynamic IP system. These companies have terrible reputations.