First of all, both SSDs and HDDs are mass storage devices. They are where your files are physically saved. They're different from RAM memory. In practice the only difference between them is how fast are files read and written. In other words, how fast the operating system will load at start up, how fast applications will open, how fast large files will be copied, and so on. If you work with lots of files over 1 gigabyte, or you simply play lots of heavy games that are full of loading screens, this is a difference that will matter a lot.
SSDs are faster than HDDs.
However, they are also more expensive per gigabyte of storage. Consequently, it's often a good idea to have both an SSD and an HDD installed in the same computer. On Windows they're just going to appear as two different drives in the "This PC" location, the C: drive being whichever drive you installed Windows on.
You'll want to install the operating system in the SSD because they're files that are accessed all the time, so keeping them in the SSD will improve your PC's general performance.
Warning: although traditional SSDs and HDDs are connected to the motherboard in the same way, via SATA port, an NVMe SSD is connected in a different way, via PCIe port. If you are purchasing one to install in your PC, make sure you have an appropriate port available first. Additionally, SATA-based drives require a connection to the power supply unit, and some PSUs have a limited number of these connectors.
