A SSD, or solid state [hard disk] drive provides a significant boost to computer performance over a traditional (platter) HDD additionally this technology generates less heat, less noise and provides for better battery consumption, but because of the cost to manufacture your typically SSD will be significantly smaller than the traditional option. A standard traditional hard drive will be perhaps 1TB or 2TB, a typical SSD might be 250GB (i.e. a quarter of a TB).
Make sure that you have considered your required HDD space before purchasing. You will have to share the hard drive with Windows, its updates, temporary/system files, program files (including games) as well as your data.
In a desktop PC, you may wish to consider two hard drives, a SSD for Windows which allows for fast start-up and general performance and a larger traditional (slower) hard drive for storage of large amounts of data. In a laptop you don’t typically have that option as there is normally (but not always) only space for one hard drive.
Images, video and installed games are the biggest hard disk consumers data wise (besides Windows and other required applications).