Notice that find found only files that were named exactly XF86Config,
rather than any files containing that string of letters. Also, find
actually tried to look in every directory on the system - including some
where you didn't have read permissions. That's why you got the
Permission denied messages.

The syntax is different as well. With find, you had to specify what
directory to search in, whereas locate automatically chose the root
directory. And you had to specify a search by name using the -name option.
You could also have searched for files using many other criteria, such as
modification date or owner. To have find search for files whose names
match XF86Config, you'd have to use a wildcard:

$ find / -name '*XF86Config*'
Like most of the command line tools, find accepts wildcards as arguments.

In general, find is a more powerful utility, and locate is faster for
everyday quick searches. The full range of possible searches would take a
long time to explain; for more details , type info find, which will bring
up the very thorough info pages on find and locate.

Determining a File's Contents

Debian comes with a utility that can guess at the contents of a file for
you. Although it is not 100% accurate, you can use the following command
to explore your system:

$ file /bin/cp
You should see something like this: