Batch processing is useful when you have a large amount of images that need to be resized to a specific size and/or file type. I LOVE this feature in Photoshop!!
1. First create a folder that contains all your images. (I usually place it on the desktop for easy reference.)
2. Open Photoshop. Go to File>Scripts>Image Processor
3. Select your folder from its location in step 1.
4. Tell PS where to place the processed images. (I usually select save in same location)
5. Specify file type you desire, pick quality level, and check the box resize to fit and fill out the dimensions you desire.
6. Press the Run button on left to start the process.
Go grab a cup of coffee while PS does it's magic!