From the NY Times:
TIP OF THE WEEK Have you ever wanted to rename a whole group of files at once? Although earlier versions of Mac OS X did not make it easy (without extra software or scripts), OS X 10.10 Yosemite can rename multiple files at the same time.
Just select all the files you want to change (like a folder of vacation pictures), and right-click the selected group to see the contextual menu; you can also hold down the Control key while clicking to get the menu. Choose Rename Items from the menu. The box that appears offers multiple ways to rename the selected files, including replacing part of the existing name, adding text to the file name or creating a new name format and numbering system that changes the camera’s numeric photo titles to more descriptive file names like “Paris (1).jpg,” “Paris (2).jpg” and so on.