Everyone I would hope would know what a normal postcard is, so what I am talking about here is more what kind of marketing message you would use with your postcard.
Given the small size of postcards they usually have a very short and to the point marketing message. You just do not have room to say much else, and so they usually have an announcement or a single statement on them.
Marketing postcards are also going to be very heavy on visuals. You need to make sure that people take notice of your postcards before they throw them in the trash, and the best way to do that is with strong visuals. Most marketing will use full color postcards, or other colorful forms of postcard printing to ensure people take notice of them.
As I mentioned briefly, postcards are going to be used when your message is short. If you have a sale coming up or something else that just needs a short message than you might want to go with postcards.
They are also going to be used when you want to have a greater level of assurance that your marketing gets to your customers. You can be certain that they will get the postcards, and if you did a good enough job on making your postcards, you can be relatively sure they will read it as well.
Postcard printings tied very tightly to a certain method of distribution. If you are using postcards than odds are good, you will also be using direct mailing. This is the chief way that you deliver your message. Now, you most certainly can get postcards and then hand them out in person. I have seen many stores that had stacks of postcards on a counter for people to pick up, and this certainly had its advantages.
Still, the way you usually see postcards distributed is with direct mailing. This lets you get your message right to your customers in a way that you cannot really achieve with most marketing, and that is why so many companies make use of it.
The main reason why you will be using postcards is that they can deliver your message to a specific pool of people with ease. The brief nature to them helps increase the number of people who are going to take the time to read what your postcards have to say.
I would suggest that every business take the time to look into how they can make use of postcard marketing. Even if you do not make it the centerpiece of your marketing, it is always nice to be able to send a concentrated message out to all of your customers all at once. Get your mailing list set up just in case, and find the right moments to get the most from your postcards.