Shells cache the commands which you have executed so as to save the time required to search your PATH every time you execute it. Type 'hash' in your terminal and you will see something like:
$ hash hits command 1 /usr/bin/which 2 /usr/local/bin/log4cpp-config 1 /usr/bin/gnome-screensaver 1 /sbin/ldconfig
which is the list of commands I have so far executed in this instance of the terminal, which is verified by:
$ type ldconfig ldconfig is hashed (/sbin/ldconfig)
Every time you execute a (non shell-builtin) command, it is automatically added to the 'cache' and lives there for the rest of the session- once you logout or even close the pseudo-terminal (like gnome-terminal) it is gone.
Another observation, the most recently executed command is added to the bottom of the list. I found this a good read: http://crashingdaily.wordpress.com/2008/04/21/hashing-the-executables-a-look-at-hash-and-type/
