SSD and hard drives

You are correct to some degree. An SSD uses Flash Memory and has no moving parts, so the read and write speeds are much faster. However, this technology is much more expensive to produce than the traditional mechanical hard drives, especially at higher capacities. Because of this, many home users do a setup that involves using a smaller capacity SSD for their operating System and the programs they use most often, and then they get a larger mechanical hard drive to store their pictures, movies, music, and the programs they use less often. This makes the things they do most often load very quickly, without having to pay for this speed for the data they use less often and/or is mostly stagnant.

You can also see http://www.myce.com/article/what-is-the-difference-betw... for more of an explanation if you like.