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Interaction design principles
First, lets start with a term interaction design
Should the close buttons position be fixed or scroll with content? Should it have a label: "close", or is a simple × symbol enough? Where to put the shopping cart icon? How fast should a modal window fade in and out? Should next screen slide in from left or right? What happens when i press on users avatar?
How should we know the answers to those questions? But there are some customs, patterns, standards and traditions we call the principles.
Here is a great article from TopTal: Boost Your UX with These Successful Interaction Design Principles