The most popular use cases for shape keys are in character facial animation and in tweaking and refining a skeletal rig. They are particularly useful for modeling organic soft parts and muscles where there is a need for more control over the resulting shape than what can be achieved with combination of rotation and scale.
The industry standard for computer animation. If you want to do animation professionally, this is the program you should focus on. 3D animation, modeling, simulation, and rendering software with an integrated, powerful toolset. Use it for animation, environments, motion graphics, virtual reality, and character creation.
A newer animation program with an interesting twist. Instead of animating the traditional way, Character Animator uses you use your web-cam and microphone to automatically animate a pre-built character in real-time, almost like motion-capture. This is a great option for beginners since it requires the least amount of animation experience.