When Intents are broadcast they will be picked up by all receivers that listen for that intent.
So, for example, when an SMS_RECEIVED intent is broadcast, it'll be received by the Messaging app, and on my phone my Phone Finder app, Missed Call, and a bunch of others that respond to SMS commands.
If an app is launched by an Intent (such as Radar, or opening a URL) and there are multiple applications that can handle that Intent, then Android either uses the default all that's selected, or pops up a selection box asking you to choose which app to use (optionally checking a box to make that choice the default).