If a user types in:
($ being the prompt for a user)
Then ifconfig will not run, UNLESS, /sbin is in the user's path.
$ echo $PATH
Will output what the user's paths are, e.g.:
As you can see, my path variable does not contain /sbin so when I type in 'ifconfig', the terminal will not look in /sbin
If root types in:
# echo $PATH
(# being the prompt for a root user)
You may get: /usr/local/sbin:/usr/local/bin:/sbin:/usr/sbin:/bin:/usr/bin
As you can see, /sbin is there, so when 'ifconfig' is typed in, the terminal will look in /sbin
By typing in: /sbin/ifconfig
You are giving a full path, so the terminal does not have a search for the file, it simply executes the program you have given it.
Hope that's clear,