Creating an Alias in Mac OS X is easy. Hold down the Option + Command keys and drag a file to some new location. Just like that you’ve got an Alias, which is, essentially, a pointer to the original file. The truth is, however, I’ve never had much use for Alias files. Symbolic Links, however, are a whole different kettle of fish.
Symbolic links are a feature of HFS+ and UFS file systems. They allow allow multiple references to files and folders without requiring multiple copies of these items. What this means is that you can relocate a library file from some application required location to somewhere else, like Dropbox, and your applications will be none the wiser. I use this trick to keep my Bento (which mandates its data file be located in the Application Support folder) to Dropbox so I can access my Dropbox data from multiple Macs.
The trouble is, creating symbolic links is a pain, requiring several terminal incantations. Not anymore. Mac OSX Hints has an ingenious Service using a bit of bash code to create your own symbolic link service. Check it out.