Leo says that all traditional spinning hard drives are basically the same. They're basically like record players, but instead of vinyl, they use spinning metal plates. Those plates are magnetic, so they can be magnetized. They also have read heads, at least one per platter, which are like the needle on the record player. Except instead of reading the grooves in vinyl, they're reading the magnetic signals coming off the spinning platter. Because it's a computer, everything is recorded as 1's or 0's, and it's very easy with magnetic material to have a charge or no charge. Those platters spin very fast. On spinning drives, the slowest ones are 5400 rpm, and the fastest are 10,000 rpm. The heads move very quickly using an actuator. The problem with this setup is that there are a lot of moving parts that can be damaged, and the spinning and moving of the heads take time. Even at the fastest RPMs, the head has to wait for the platter to come around to read each sector of data. That time it takes is called "seek time."
Solid state drives don't have any moving parts. Instead, they have little cells that can be electrically charged. It's almost identical to the way SD cards work that are used in cameras. They are more reliable, and there's no seek time because it seeks at the speed of light. It can find data anywhere inside that solid state drives instantly. This reduces the amount of time to get the data. This makes SSDs a better choice because of the speed and reliability. Nowadays, most computers come with SSDs. The overall speed of the computer is measured in a lot of ways. The processor may be very fast, but the bottleneck then could be the spinning hard drive.
Check out the original question from July 23, 2017 here.