SSDs do the same thing as a traditional hard drive, they store data. The difference is that they are 1. more expensive, 2. much faster, and 3. more durable. They have no moving parts so they can be shaken around and dropped and still function where a traditional hard drive is much more likely to fail. As far as performance goes, the most noticeable difference is boot up time, instead of waiting between 45 seconds-3 minuets, you can boot in around 20 seconds. Also load times of anything on the SSD will be faster.
Yes, the speed increase is worth the price.
The only drawback to this is price. It is expensive if you need a lot of storage. The fix here it to get a SSD (maybe 128 or 240GB) for the OS and a few applications, and then use a HDD for data.