Well, now Tesco has got an API, so that's probably not good enough anymore. And, following Einstein's assertion that if you can't explain something simply then you don't understand it properly, I thought I'd have a go at explaining it.
First things first: API stands for application programming interface. Second things second: it's a way of giving applications (typically built by someone other than you) controlled access to a set of data you own and generate. So, for example, the Google Maps API lets you embed maps and play around with them or the Amazon APIs let you make money from promoting Amazon products.
And APIs mean that the functionality of a site can be extended by people other than the data owner. For instance, there's a company called Moo that will make lovely little business cards and stickers for you based on the pictures in your Flickr account. And you don't have to download them from Flickr and upload them to Moo because the Flickr API grants Moo the access it needs to extract the data and pictures (once you've signed in and given permission).
It's API as a win-win for everyone. It gives Moo a basis for a business, adds value to your Flickr account and makes getting some nice stickers easier for you. Does that all make sense so far? Good. So, why does news of Tesco's API strike me as significant? Partly it's because a very British, very bricks-and-mortar company is doing something very digital. I know it's a huge web retailer and has built solid, well-designed stuff that always works (as it should), but Tesco has never really played in the fancy, open world of Web 2.0 and all that gubbins.
Until now, Tesco has seemed as monolithic and controlling as all the other big retailers and brands: unlikely to let people play around with its data. And it's not just the API announcement that's interesting, it's the way Tesco has done it: not with big corporate fanfare, but quietly, into the developer community via blogs and mailing lists. Tesco is behaving in an appropriate manner for the community it wants to address.
There's even going to be an "open innovation day" called T-Jam, which feels much more like the kind of thing Yahoo! would do. And perhaps that's the most important thing to understand about APIs: it's not just about the technical stuff, it's the attitude that it symbolises, acknowledging that being a supportive and generous part of a business ecosystem can be good for everyone involved.