Windows: What is the difference between installing for all users and current user?

In a nutshell...

Install For All Users would be used when you want the program to be "active" for anyone who logs on to the computer with their personal account (if there is more than one login). A good example would be an antivirus program, that you would want to be available while any user is logged in.

Install for Current User is normally used under an Administrator account for something that handles disk operations and other admin-type functions, such as Acronis True Image, a keylogger (to track what non-admin users are doing while logged on), and other disk-related or sensitive software.