Literally defined, a "killer application" a.k.a. a "killer app" is software that is revolutionary and popular (must be both).

In layman’s terms, a killer app is the software giving you the reason to use a computer (or possibly a specific computer) in the first place.

The history of killer apps basically have two generations, that being pre-internet and post-internet.

Pre-internet

In the days where personal computers didn’t connect to any outside networks (other than BBSes), there were two primary applications people used at home. The word processor and the spreadsheet application.

When most people think of word processing they think of Microsoft Word. However this was not the "big" word processing app back then. Most people at home used WordPerfect. And being that this was in DOS it looked rather terrible because there wasn’t any WYSIWYG. But it did work and work well. If not WordPerfect they used an app called Text in DeskMate.

The Apple Lisa (predecessor to the Macintosh) did in fact have a word processor with WYSIWYG called LisaWrite. However few people owned the Lisa machine because it was geared towards business customers and cost $10,000 for the system.

In the word of spreadsheets most people think of Microsoft Excel, but the killer app in pre-internet days was Lotus 1-2-3. On Apple systems as well as other 8-bit systems the popular spreadsheet app was VisiCalc.

A primary effort was put on publishing and home business in pre-internet times, because said honestly there wasn’t much else you could do with a personal computer unless you were a programmer.

Post-internet

The primary application, that being the killer app, in post-internet times is something you’re probably using right now – the web browser.

The web browser by itself does nothing. It literally is a software version of the yesteryear dumb terminal. Without internet connectivity it is useless. But with connectivity it is your gateway to the internet – just like a dumb terminal is useless without its connectivity.

There has been nothing introduced to computers since the web browser that qualifies as the application people use most on their computers from the moment they turn them on. It has even superseded word processing and spreadsheet applications as most-used.

When you think about it, is there an app you use more than a web browser? Probably not.

Additionally, all the spreadsheet’ing and word processing you do can be done via the web browser. This "dumb" software is the one tool you cannot possibly be without if you use a computer at all.

The future

What will the next killer app be? Hard to say.

But what is being said right now – and has been said for quite a while – is that web browsers are still rather terrible and desperately need to change.

Google Chrome is an example of what direction the internet might be heading towards. The direction it indicates is both cool and scary and the same time.

The cool part: It is a browser that works near-perfectly with services provided by the #1 web site in the world.

The scary part: It heavily leans towards a proprietary internet that shuts out other sites in one fell swoop. Granted, nothing works better than proprietary software – but that’s not what the internet is about.

It’ll be interesting, no doubt there.