Your computer probably uses a variety of applications including a word process, spreadsheet, accounting software, etc. These applications normally store their data in a variety of places on your computer. With a proper backup and archival system you need to save the working data from all of your applications. To make this process easier you can create one working directory (or a limited number of directories) where your applications store their data. In my case I have a directory for personal data (letters, resume, etc.), photos, business (all my business notes), etc. When I backup my data I copy these directories to a new backup folder on the external hard drives.
Many applications by default store their working data deep in the system directories on your computer. These can be more difficult to find and you may forget to back them up. You may want to create a directory called c:/Records (for example) and have a folder for your accounting data, another for your working documents, and another for spreadsheets, etc. You can then copy this entire folder to your backup drive to protect your data.