A postal card is a type of post card. The difference between a postal card and a normal post card is that a postal card is issued by the post office, and has postage preprinted on it. They are about the same size as a standard postcard, and are used in much the same way, with the exception that the destination and return address are placed on the obverse side, along with the "stamp", and the entire reverse side can be used to write a message. In the United States, posal cards are issued in both definitive and commemorative issues. The post office has been releasing postal cards since 1873, and they can still be purchased for two cents over face value at any post office.
Postal cards are not a common item, falling as they do in the area between letter and postcard. Postcards, as a class, are not usually sent for utilitarian reasons. People usually send postcards because they are on vacation, or because they want to send someone something brief and cute. Postcards, as decorative items, are probably kept almost as often as they are mailed. Postal cards are rather boring looking objects, despite the great effort put into the design of the "stamp". And, unlike a letter, postal cards would probably not be most people's chosen method of writing an in-depth, complicated letter, given that even for those with very neat handwriting, they only hold a paragraph or two of writing. For this reason, postal cards are not often used.
For the frequent mailer, or for those who are exhaustive in their approach to postal products, the postal card can be a nice addition to the arsenal. One of the major problems with post cards is that their colorful designs may distract from the message you are trying to write, as well as the fact that they usually don't offer much room for writing. A postal card allows a blanker slate. In addition, the smaller size can be a blessing, as it allows a more pithy message.
I would suggest for those who live in countries where postal cards are issued, that buying a small pack of them, and finding usages for them, may be a rewarding experience.