Simply, it’s another tactic marketers use to reach their target audience.
The longer story: Mobile marketing is the use of cell phones and other mobile devices to communicate a brand message. It has emerged as the most complimentary form of marketing due to the way it gives legs to all other mediums. For that it is now a key component in any integrated marketing strategy.
Examples could include: Text message campaigns, iPhone development, mobile advertising, mobile web sites, custom software, ringtones, wallpaper