"The only intuitive interface is the nipple. After that it's all learned."
When we say something is intuitive, it means that something easy to use. We don't have to spend time reading instruction manuals to figure out how it works, we just use it, and it works. It's simple.
Intuitive is a quality that every designer strives for in their work. However, you can't strive for a goal you don't understand. So ask yourself, what makes something intuitive?
The answer, actually, is different for every single person. What's intuitive for me, might not be intuitive for you. That's because intuition is the sum of your past experiences. It's everything you've ever done in your life, and your life is different from mine. That's what the quote about the nipple is about. The only intuition that we're born with is how to get food. Everything else is learned.
Intuitive interfaces seem easy-to-use because they're based off of something you've already done in the past, something you've already learned how to do.
Take the original interface concepts from the desktop PC for example. You have a desktop, trash, files and folders, just like you do at the office. This metaphor makes the interface 'simple' to anyone who's ever been in an office environment, because it draws on peoples past experiences.
The interesting thing about what we consider intuitive is that it changes over time. For example, kids today will grow up having never sent a physical letter by post. So the metaphors that made email 'simple' for generation of users are lost on the newer generation. The 'C.C' and 'B.C.C' fields in email are a prime example of this. Who's actually ever made a physical 'carbon copy' or 'blank carbon copy' of anything in the past 20 years?
The base experience for the new generation now becomes the digital interface itself, once a metaphor of something else. This generation is referred to as being digitally native. And the transition towards creating interfaces that make sense for these people is already underway.
Just look at iOS 7 or Windows 8. These interfaces have buttons don't look like buttons, there are random flat tiles everywhere, there's no desktop, no trash, and hardly any files or folders.
Does that make these interfaces less intuitive? Well, like I said, that depends on who you ask.