There are a million articles online about design. Here are a couple of my favorites to get you started.
This contains my absolute favorite summary of what design really is:
Design, for my purposes, is about making tradeoffs. Let’s design a trashcan for a city street corner, shall we? Let me give you some design constraints. It has to be pretty light, because the dustboys, er, sanitation engineers come by and they have to pick it up to dump the trash in the garbage truck. Oh, and it has to be heavy, or it will blow away in the wind or get knocked over... It has to be really big. People throw away a lot of trash throughout the day and at a busy intersection if you don’t make it big enough, it overflows and garbage goes everywhere... Oh, also, it needs to be pretty small, because otherwise it’s going to take up room on the sidewalk... Ok, light, heavy, big, and small. What else. It should be closed on the top, so rubbish doesn’t fly away in the wind. It should be open on the top, so it’s easy to throw things away... Notice a trend? When you’re designing something, you often have a lot of conflicting constraints. In fact, that’s a key part of design: resolving all of these conflicting goals.
Starting in about 1992, these interfaces started to disappear, to be replaced with a new invention called tabbed dialogs:
Tabbed dialogs are a great affordance. It’s really obvious from this picture that you have six tabs; it’s really obvious which tab you’re on, and it’s really obvious how to switch to a different tab. When Microsoft first usability tested the tabbed dialog interface, usability went up from about 30% to 100%. Literally every single testee was able to figure out the tabbed dialogs.
The interface design must assure that a user can never inadvertently fire the ejector seat when all he wants to do is make some minor adjustment to the program.
I can think of a half-dozen applications I regularly use where the ejector seat button is inexplicably placed right next to the cabin lights button. Let's take a look at our old friend GMail, for example:
Every few days I accidentally click Report Spam when I really meant to click Archive.