Many computers come with a solid-state drive (SSD) in addition to the regular hard disk drive (HDD). For example, one configuration might be a 256GB SSD and a 1TB HDD. The SSD has no moving parts and is much faster than the HDD. An HDD has mechanical parts, is larger in size, and is slower to read and write data. An SSD is more expensive than an HDD of the same capacity.
The benefit of having both an SSD and HDD is getting better performance than a system with HDD only, at a better price and with more storage space than a system with SSD only.
Typically, the operating system (OS) is installed on the SSD. With the OS stored on the SSD, your PC boots up and is ready to go much faster than if the OS was on the HDD.
You can install applications on either the SSD or HDD -- just browse and select the preferred storage location during installation. It doesn't have to be in the Program Files folder.
The SSD will have better performance, so it's best to prioritize the programs you use most often and install those on the SSD. Especially if you're using any resource-heavy applications, such as those for video editing or gaming; these should be installed on the SSD instead of the HDD.
Make sure to store all your media files (photos, music, videos, etc.) on the HDD so that they don't take up space on the SSD. The higher performance of the SSD won't make much of a difference when accessing media files.