Resources are external files (that is, non-code files) that are used by
your code and compiled into your application at build time. Android
supports a number of different kinds of resource files, including XML,
PNG, and JPEG files. The XML files have very different formats depending
on what they describe. This document describes what kinds of files are
supported, and the syntax or format of each.
Resources are externalized from source code, and XML files are compiled into
a binary, fast loading format for efficiency reasons. Strings, likewise, are compressed
into a more efficient storage form. It is for these reasons that we have these
different resource types in the Android platform.
This is a fairly technically dense document, and together with the
document, they cover a lot of information about resources. It is not necessary
to know this document by heart to use Android, but rather to know that the
information is here when you need it.