Traditional Hard Disk Drives are usually the bottleneck for any computer system, primarily because of the nature of how it operates. The read/write head needs to be in precise position with the platter to locate and access a specific sector, thus the platter also needs to spin as fast as possible.
Then comes Solid State Drives or SSDs. The main difference is that there are no moving parts, thus Solid State. No moving parts means the data is accessed instantaneously. Most SSDs will give you 6Gbps, which is actually the limited of current SATA interfaces, compare that with SATA HDD which is about 3Gbps.
If you want to upgrade your computer’s storage to SSD, most SSD sites offer a list of compatible systems for their SSD drives.
Take note that most SSDs do not offer the very large capacity that HDDs have now achieved. Most common SSDs are 240GB to 500GB.
If you have ever replaced or installed an HDD, it will be the same as for an SSD.