When you are sitting at your home computer and you print something out, you are creating a print job which your computer sends directly to your printer. When you add a network to the mix, that same print job can take several extra steps, and it is often these extra steps that lead to a misunderstanding of printing problems.
In the above diagram, you can see that there are several steps that a print job can make between a computer and the printer. The first and most basic addition to the print path is the print server. A print server is added to the path to take the load of managing print jobs off the individual clients, and to keep the connections to the printer simple. The print server will manage a print queue that manages the jobs until the printer is ready to print them. For example, lets say that you have 20 people using one printer and they all decide to print at once. Without a print server that queues those jobs up in order and sends them to the printer one at a time, the printer would be deluged with jobs possibly resulting in an error. A print server is the main component that allows multiple people to share a printer.
The next main addition is some kind of external application. This could be a web app of some kind or in a business environment it could also be some kind of mainframe style application. This adds the possibility that the print job, either in whole or in part, is generated outside of the client workstation, and the user only initiates the job. This can add a layer of complexity as well. For example, let's say you made a purchase on your favorite website, and you want to print an invoice. That invoice will be generated by an external application which then uses your local computer as a conduit to your printer. This is very similar to many business applications, however it may be that the application is connected to a print server instead of using your computer to make the connection. Additionally, it may be connected directly to the printer itself, functioning as a print server itself. The configuration may vary from app to app.
So, basically there are two (or three paths depending on how you count it) that a print job can take from a computer to a printer. The blue path represents the most common path, and you can remove the external application portion if you are dealing with applications that are only locally installed. Note: Web apps are external apps. The green path deals with an external app that has it's own path to a printer and does not use your normal connection.
