Suppose every time you update your status on Facebook, you also want to post exactly the same content to your Twitter account. And every time you upload a photo to Instagram, you want it to be saved to your Dropbox as well. How would you do it automatically? Here is when IFTTT comes in. It is short for "if this, then that". Fairly intuitive. You can put whatever you want in "this" and "that".
Channels are Twitter, Facebook, email, Google Drive etc. Triggers are what can be used to prompt an action, such as "New status message", "New photo post", and "You are tagged in a photo" from Facebook. Action is the work that IFTTT undertakes as a result of a Trigger. Examples are "Send me an email" from the email channel, "Create a status message" from Facebook, and even "Update device wallpaper" from the Android device channel.
Now, to get our first job done, we only need "if New status message, then Post a tweet." Just go to ifttt.com, select a Trigger from a Channel, and connects it to an Action from another Channel. That's all you have to do to automate your apps or smart devices. The user interface is extremely intuitive and easy to use. It's just like putting Lego parts together. You don't need to know any technical stuff to make it work. And the service is extremely reliable, I've never experienced any downtime myself.
Although my experience at their website is very positive, I don't think their Android/iPhone app is of a equal match. The app's interface is kind of primitive and it took me some time to find what I want. Generally it takes 2-3 steps to go to a specific place. It's just not as good.
And for now the whole experience is fragmented, say if you want to connect your Facebook and Instagram accounts, you'll have to go to IFTTT first, and grant it authorization to your accounts. It would be a much better experience for us users if we can just do it in FB or Instagram, thus making it invisible, yet everywhere. Actually it's what they are trying to achieve this year and I know they're developing their SDKs.
I'm currently using the BMW's channel on IFTTT. I have my BMW send me an email of the map to where I just parked. And I also connected it to hue, so whenever I leave my home with my car the lights will be turned off.
I also have a device (BloomSky) that measures the weather and takes a picture of the sky every 5 minutes in my home, I connected it to my Android phone. Whenever there's a new image, my phone will set it as the wall paper.
There hundreds of Channels out there and people's imagination is unlimited.