PDB stands for Palm Desktop Database. The file format can contain several kinds of information but often is used to store e-books. The format only is compatible with devices manufactured by Palm, and converting PDB e-books to a more universal format, such as PDF, is a good idea if you plan to use the e-book on other devices.
The PDB file extension is used by software unrelated to Palm, including many financial management applications. In the context of converting to PDF, however, PDB almost certainly refers to a Palm Desktop Database file that stores various data, including Word documents, such as ebooks. To know for sure what the PDB file contains, open it in a text editor. If it is a file from a Palm device, it should contain header information, near the top of the file, that identifies it.
PDF stands for Portable Document Format and is used by Adobe, the developer, as a file format for various Adobe software. The format is supported by many other applications and devices, such as e-book readers. PDF files can contain data ranging from text and forms to images and are a popular format for e-books.
The conversion process is usually from PDB, which is a proprietary format, to PDF, which is a far more universal format. There are several free applications, such as Calibre, that can convert a PDB file to a PDF file. Calibre is a useful format converter because it also can interface with many e-book technologies. Online-Convert and DocsPile are two free online converters that can handle the process, as well.
E-book readers are a popular and growing way to read books without the need of paper. They operate on the same principles as an etch-a-sketch, pushing the needed pixels to the surface while allowing the unneeded ones to remain hidden. PDF is a format supported by many e-book readers. Handheld devices, such as those made by Palm, also can be used to read e-books, though they are not as well suited to the task, chiefly because the battery life is so much better on most devices made specifically for reading e-books.