In a true Liquid layout, your content expands and contracts to fit your user's browser window in a meaningful, calculated and intelligent way. So it's more than just setting your column and container widths to percentages.
Done well, this can result in a increase of perceived quality. Done poorly, it's a usability nightmare.
Going Liquid is a huge pain the rump. The pain is worth it though if the topic/client/product(s) you are building the site for have a strong visual quality to them (think summer blockbuster film site), require a certain fit and finish, or if it needs to display large chunks of data.
Note: I'll update this a bit later with links to good examples and citations for my claims