Question

What is a BroadcastReceiver? What are its uses and how can I use it?

Answer 1

Start by reading the documentation. Also, copying from Application Fundamentals:

Broadcast receivers

A broadcast receiver is a component that responds to system-wide
broadcast announcements. Many
broadcasts originate from the
system—for example, a broadcast
announcing that the screen has turned
off, the battery is low, or a picture
was captured. Applications can also
initiate broadcasts—for example, to
let other applications know that some
data has been downloaded to the device
and is available for them to use.
Although broadcast receivers don’t
display a user interface, they may
create a status bar notification to
alert the user when a broadcast event
occurs. More commonly, though, a
broadcast receiver is just a “gateway”
to other components and is intended to
do a very minimal amount of work. For
instance, it might initiate a service
to perform some work based on the
event.

A broadcast receiver is implemented as a subclass of
BroadcastReceiver and each broadcast
is delivered as an Intent object. For
more information, see the
BroadcastReceiver class.

Finally, read in Common Tasks how you can utilize BroadcastReceivers to listen for messages and set alarms.