There are various sorts of things that the system can generate for printing. They are as follows:
Reports - these are invoked for the Reporting|Reports screen. Typically a report has some input parameters (eg the from and to dates, the product type, etc). As well as printing the report, you can also generate a PDF file, or export the data for use in a spreadsheet. Reports can be give a 'level', and only users with a 'level' equal to above that will be able to run the report. A number of standard reports are included with the system distribution, more are available from the community resource. If you want/need to modify or create reports, this is possible but need technical skills - see here.
Forms - these are what they sound like - forms that the system can generate that can include merged-in information about the current patient, customer or supplier. You can modify or create these with a standard word processor. See here for further information.
Letters - these are not quite what they sound like - in fact, they are just forms with input parameters. They are thus used where it is necessary to show information that is not available from the system. For example, if we have a 'Health Check for Purchase' form, then although the system can automatically fill in the patient details, comments on items like Eyes, Ears, Teeth & Mouth, Chest Auscultation, etc need to be provided by the vet. Rather than printing the form and then filling these in, the system can prompt for this information and enter it on the form. Since you can use macros when entering the prompted-for data, you can quickly enter standard information.
System Documents - these are the things like invoices, receipts, etc that the system generates. The system distribution includes these, but it may be desirable to modify them. If you need to do this, see here.