What are Intent Filters exactly?

I think it's well documented here. Summarizing, when (e.g.) you pass an intent to Context.startActivity, or Context.startService, or broadcast it with Context.sendBroadcast, etc, what activity or service (or, what set of broadcast receivers) is the intent delivered to?

Answer: if the intent is "explicit", i.e., it has the component name field set, then it's delivered to the target component it designates -- this typically happens in communication within an application.

Otherwise ("implicit" intent, as is typical for communication between applications), Android must determine the best activity or service (or set of receivers) on its own. How does Android do that? That's where filters come in, and I quote:

It does so by comparing the contents of the Intent object to intent filters, structures associated with components that can potentially receive intents. Filters advertise the capabilities of a component and delimit the intents it can handle. They open the component to the possibility of receiving implicit intents of the advertised type. If a component does not have any intent filters, it can receive only explicit intents. A component with filters can receive both explicit and implicit intents.

The web page in question continues with many details and, at the end, a complete, simple example, and I think it would be redundant to copy and paste them here.

Simply put, Intent filters are a way of telling the OS how to launch/communicate with the different activities/services/broadcast receivers in your app. So for example, if you want links that start with http://www.mycompany.com to lead people into your app, an intent filter is the way to accomplish that. Once its setup, anytime someone clicks on a link that starts with that (in any app) the user will be presented with the option to use your app to view that page. You've probably seen this with youtube urls. Likewise, if you want the 'share' link commonly seen in many apps to list your app, would use an intent filter to do that.

hope this helps...