When you are working at one of the Macs, you should save your work in a folder with your name on the 'Scratch' drive. You can make a new folder by holding down the 'control' key while clicking where you want the new folder to be, and selecting 'New Folder' from the menu that appears. The 'Scratch' drive is a 'local' drive (meaning it's stored on the hard drive of the computer you are at, rather than on the network), so you'll need to either transfer your work via the Media Server, copy to a USB Drive or burn a CD if you want to work on the same files at another computer. The 'Scratch' drive is the only place on the Macs hard drive that you should save work. 

You should always make backups of your work (by copying to a USB Pen, for example). As everyone can use the 'Scratch' drives, you can't guarantee your work will be there still next time you use that computer.