The announcement that Steve Jobs is stepping down from the helm of Apple is sure to send waves of surprise through Silicon Valley. To understand why, consider this:

In addition to the basic fact that he was head of the largest technology company in the known universe, Jobs did, oh, just a few other things.

One of them involved perfecting and popularizing the graphical user interface -- you know, the thing that meant you didn't have to type gobbledygook onto a green screen to use a computer.

The other you might recognize by looking down at something in your pocket or briefcase, assuming you have a smartphone or tablet. Although Jobs certainly didn't invent mobile computing, the devices he created have arguably furthered the field more than any others.

So what was Jobs's most important contribution? Let's take a look.

First, there's the graphical interface. Jobs himself didn't have the idea for this; rather, ideas for it came from a 1979 visit to a Xerox researchfacility. But the Macintosh, introduced in 1984, refined the idea into a form that could be used by the average consumer.

Prior to the graphical interface, people using computers had to input textual commands. This, needless to say, was not what one might call 'user friendly.' Jobs himself described it this way: 'The special incantations you have to learn ... are the 'slash q-zs' and things like that. ... [People are] not going to learn slash q-z any more than they're going to learn Morse code.'

The system of using a mouse and moving things intuitively on the screen has been used in most later consumercomputers -- including, of course, Windows. Without it, computers would not likely have become as popular or usable as they are.

Next up: the mobile computing revolution. Sure, there was the BlackBerry before the iPhone. And sure, Android can lay claim to the 'most popular' title. But the iPhone and iPad have revolutionized the technology landscape.

The popularization of consumer mobile computing is changing the way we interact with technology. Tech is now more a part of us than ever before -- almost an extension of the human body at this point. Jobs went back and forth over the years on whether technology could really change the world. But it seems that mobile devices might be doing that -- for good and ill. On one hand, we are more in touch than ever before and have major computer and information power literally in the palm of our hands. On the other, the extreme association between technology and our physical space is raising questions about surveillance capabilities and our privacy. These are major societal issues, here.

Of course, the question remains whether either of these things -- like fire and the wheel -- would have been invented and popularized with or without Steve Jobs. But the fact of the matter is that he's the one who accomplished it.

So what was Jobs's biggest contribution? In your Digits blogger's opinion, it's the graphical user interface, because consumer use of technology was necessary before the mobile revolution could take place. But readers, what do you think? Was it one of these two contributions, or something else?