A transciever is a small box-like device that you connect to your AAUI port. At it's other end is either a BNC or RJ-45 port. Networking two Macs can be done using thinnet transcievers - less expensive, but limited in expansion capabilities. Networking more than two Macs usually involves a network hub. Small networks can operate using a Farallon EtherWave transciever acting as a small hub. There are many different ways to establish an ethernet network. Throwing PC's into the mix makes it even more complicated. Luckily, the Macintosh makes it possible in a matter of (several) minutes, as opposed to days. Consult your local knowledgeable salesperson about how your ethernet needs can be solved.
One great trick that works almost as well as ARA is to utilize your ftp space at your internet service provider. If you have space reserved (rented) with your provider for web pages, simply upload files to your ftp folder. Those files can then be accessed at any time from anywhere. Of course, this works best when there is someone at each Mac at both ends of the transfer, but it also saves one Mac from having to be a dedicated server.
Much of the information contained herein has been extracted from Elliotte M. Harold's incredible FAQ lists. We thank and commend him for his invaluable service to the Macintosh world.