There are two major types of stock photos: free stock photos and royalty-free photos. Free stock photos are the ones that are available for download on the internet. You can use them in your projects, but you cannot sell them or redistribute them. Royalty-free photos, on the other hand, can be used by anyone and they don't have to pay any royalties or fees to use these images.
Stock images are the most popular type of royalty-free images. They are usually sold in bundles, and they have a wide variety of topics. Some stock image sources allow you to download them for free while others charge a monthly or yearly subscription fee.
Royalty-free images are not exclusive to one source. They can be found on various sites like stock photo providers, as well as through websites like Flickr and Wikimedia Commons, where you can find Creative Commons licensed photos that you can use for free under certain conditions.