An ad is a commercial or a print promotion for some product or service. In other words, an ad is an advertisement.
When a company uses print or broadcasting — newspapers, magazines, the Internet, or television, for example — to attempt to sell more of something, it's an ad. Though the word ad is an informal shortened form of advertisement, it's become more commonly used than the original. Those in the business of advertising have objected to this slang form ever since it was first used, around 1840.