The NAND cells have a limited number of program-erase (P/E) cycles. What exactly is an "erase" from the prespective of a storage device? The OS is just tell the drive that the cell is no longer needed. Being the case, the drive is free to write on the cell when needed. With HDDs, that was it... an "erase" sector was just marked as ready to use (aka writtable).
With SSDs, there is a minor limitation with these "erased" cells/pages. When the controller wants to use this page again, it has to clean out the page first through a read-modify-write action instead of just a single write action. To help prevent this, there is a command call TRIM which tells the controller to clean out these dirty pages when it can before you need them. Here is a more in-depth article: http://www.anandtech.com/show/2738/7
However, this is not a major concern for consumers anymore as most OSes and drivers support TRIM now. In addition, all SSD companies are aware of this limitation and program smarts in their controllers to help prevent this issue.
As for your SSD wear concern.... don't worry about it. Most users write less than 8GBs a day.... at that rate, the NAND will last
Here are one-off real-world test: