Stock images break down into two main types, royalty-free and rights-managed.
For royalty-free images, you get nearly unlimited use. You can use the image in virtually any application, for as long as you like, in as many different projects as you like, as long as you comply with the terms of the license agreement. The image is available to use from when you purchase a license. Following payment of the license fee, no additional royalty payments are owed.
With rights-managed images, your right to use the image is typically restricted, with limitations placed on things such as duration of use, geographic region, industry, etc., as established by your license agreement.
Royalty-free means that once a license fee is paid, the images may be used many times without paying additional fees, but the initial license is necessary to protect yourself and your clients. When you license a royalty-free image, you can use it in nearly any application, for as long as you like, according to your license agreement (although some kinds of uses do require an extended license). The cost is often based on file size, the number of permitted users as well as other factors.