To get more information about a file, select the file (you can also select multiple files) and press .
This will bring up an information dialog such as the one below:

If you selected multiple files, multiple information dialogs will appear.
If you wanted to show a summary of the selected files (information about all of the selected files), press . This will bring up a summary dialog like below:
Another useful way to use the information dialog is to press . This allows you to keep the same information dialog open while switching between different files. When you click a different file in Finder, the information dialog is updated with the newly selected file’s information.
[Submitted by Josh Lockie from MacWorld]

Luke