Trim Command: http://en.wikipedia.org/wiki/TRIM
Since SSD storage has limited life (vs HD platters), SSD controllers implement many techniques to optimize writing data. These techniques can/will lead to significant degradation of performance if the SSD is not "cleaned up". A full and totally accurate explanation is quite long, hence the wiki refernece.
Best SSD's for the Money: http://www.tomshardware.com/reviews/ssd-solid-state-nand-reliability,2998.html
The article on SSD is refreshed often and allows you to choose based on budget, desired performance, capacity requirements, etc. Its a must read if you are new to the market, or buying an SSD any time.