These principles demand that an application should be consistent and in accordance with the users' expectations. A lot of standardization work is done to achieve these goals.
These principles make applications simpler to use because users have fewer rules to learn. Thus, they can learn more effectively, as there are fewer exceptions to the rules, and can train procedures more often. In addition, users can utilize their computer and real world experience and transfer existing knowledge from one application, procedure, or screen to another, and also from the real world to the computer system.
A consistent application and user interface always follows the same general principles, the same interaction principles, uses the same terminology, and so on. Consistency can also be extended over several applications, a whole operating system, or even across computer platforms.
Note, however, that consistency is not a goal as such – it is just a "rule of thumb" in order to achieve simplicity. There are many cases, where users do not care about consistency, or do not even notice inconsistencies. Do not enforce consistency, just because it is a widely accepted principle and a good foundation for a "theoretical argument," if users behave differently and find it easier to disregard it. Often, only user tests can provide a final answer to the question of which solution is the best one for the users.
Conformity to user expectations demands that an application behaves as users expect it to do. This principle goes beyond mere consistency, because it is not restricted to the computer systems but also connects the application to the real world.
Note that user expectations can vary largely, depending on the background and learning history of your prospective users. Computer literate users will expect that your application conforms to well-established interface standards, while beginners who are domain experts will expect that your application will behave similarly to their business practices.
In summary, you can apply these principles by doing the following:
These principles require you to adhere to standards. Yes, it is great that you are a creative developer, but there should be limits to your creativity. Do not follow your personal taste, where generally accepted standards exist. Your users will acknowledge this and be grateful that you made life easier for them.
Source: Simplifying for Usability