First, business cards have been all the rage for years. I'm not saying you shouldn't use them; I am saying that postcards will get the news out about you and what you do much faster than a name, address, phone number and perhaps, "author" printed on the business card will get you.
Most of my postcards have these features:
On the Back of the Card
A synopsis. You always want a short synopsis of your book no more than 3 paragraphs long.
A QR code. I don't always used a QR code, but I am using it more often than I used to. A QR code does wonders (it's actually a postcard add-on) for anyone that takes a picture of it with their SmartPhone. In an instant they will be taken to a site of your choosing. My QR code goes to this site.
Get your QR code at: http://www.the-qrcode-generator.com/#/. There are many free options on the Internet.
I sometimes include blank lines near the bottom back side of the postcard. This space is good to write a special note to the person you are giving the card too; something personal like, I hope you enjoy the book.
If you're going to be mailing your postcards, you will only have half the space in back, so keep in mind that the synopsis might be only one captivating line, and you probably will only have room for the date, time and place of your event, and probably not the QR code.
On the Front of the Card
I always include the book cover. This fills up the entire card.
Postcards are much harder to lose than a business card and provide your future reader with enough information to get online and learn more about your book. Postcards also make great bookmarks and are great for book signings and speaking events. I pass mine out during presentations, and always have them at the table when I'm doing a book signing.
If you have any questions about using postcards that I haven't mentioned here, don't hesitate to ask.