Scan rate is the speed at which a page of paper is scanned. Scan rates are usually set at pages per minute. or seconds per page. Scan rates can become very important when you are sitting and waiting for a document to scan or if you have many documents to scan, especially when you use the flat bed type of scanner for multiple documents. Just think of how boring and time wasting feeding on document at a time into a copy machine is and you will get the picture on scan rates. Another important feature to understand is the footprint, which is the size of the scanner and the room it will take up on your desk or computer furniture.
Flatbed scanners have the largest footprint, but are often worth the space.
Ease of installation and use is another important consideration. The parallel port usually has a printer attached, which must be detached and reattached unless you have a bypass device. The serial bus, which should be the new Universal Serial Bus or USB port or you have an out of date scanner. Of the three the USB port is the best option. USB or Universal Serial Bus is a hardware interface for peripherals.