Assuming you’re using bash:

Launch Terminal

% pico ~/.bash_profile

Add a line with a variable followed by whatever command you what it to perform, such as:

webroot='cd /Volumes/apache/webroot'

CNTRL-X, then press ‘y’ to save the file and exit edit mode
now restart bash_profile;

% source ~/.bash_profile

That’s it!
Now anytime you want you can launch Terminal, simple type % webroot, and you’ll be taken to ‘/Volumes/apache/webroot’

Now will all the typing this will save you, there’ll be more time for beer at the end of the day.

Let’s say you often go to /Volumes/development/depot/mainbranch. Let’s create an alias to that called ‘mb’ for ‘mainbranch’ (this can be whatever you want, of course.
Simply launch terminal.
Type: alias mb=’cd /Volumes/development/depot/mainbranch’
Done!
Now just type ‘mb’ at the command prompt and you’ll be taken directly to that directory!
Sweet!

Note, you can also do the same to launch applications:
alias tx=‘open /Applications/TextEdit.app/’