So you heard the buzzord SSD hardDisks. or Solid State device HDD. well, they are faster due to inherited silicon business. A Normal HDD rotates at anywhere between 7500rpm to 1500 rpm to achieve good speeds that we have known till now. But SSDs take the things to silicon level where HDD is stored in silicon memory. Just like your MMC, SD camera/gadgets cards. These have ultra low latency (of the order of nanoseconds) which is comparable to RAM. where as commercial HDD rotation gives access time of the order of milliseconds. so the speed difference is obvious. But SSDs are not as fast as they should have been as per the theory.
there are potential memory problems or in other words, switching states issue which decrease the performance of SSD drives, though still faster than rotational HDD. This post is just about the basics, watch the video, you will get comparisons in action.
Now you know why Apple MacBook Air Rocks 😛