What is AirDrop?
AirDrop is a technology that allows you to send files wirelessly to someone located physically nearby, even those not on the same network as you. It's much faster than using a USB flash drive. Best of all, it's built into every Mac (sorry, no Windows equivalent is available at this time). The only limitation is range: you must be within wifi range of your AirDrop partner (typically 80-120 feet).
How do I use AirDrop?
Using AirDrop is incredibly simple: begin by clicking on the Finder icon on your Mac (the very first icon in your Dock, located at the bottom of your screen):
Then, click on the AirDrop icon in your sidebar. Your Mac will start broadcasting itself as an AirDrop recipient, and will start looking for other AirDrop recipients.
In the first screenshot above you'll see my window (with my computer located at the bottom, and my co-worker's computer above mine). In the second screenshot, the opposite is shown in my co-worker's AirDrop window.
To send a file or folder, simply drag and drop the item from your Finder (your Desktop, or even something located on a server) onto your co-worker's computer. He/she will receive a notification that you're attempting to send something to them:
In the above example, I'd click "Save" to accept the file and download it into my Downloads folder. Voila! No file server nor flash drive needed to transfer files.
Note: you'll want to wait for the transfer to complete before putting either computer to sleep, or closing the AirDrop window.