Smartphone. The smartphone started as a hybrid love child of a feature phone and a PDA. It has since evolved into a mobile device that is virtually like carrying a small PC in your pocket. The smartphone makes voice calls--hence the "phone" part. The "smart" part comes from being able to sync and carry vast amounts of data including your contacts and calendar, and the fact that the device can surf the Web. With the addition of apps, smartphones have access to tens or hundreds of thousands of programs ranging from frivolously silly like Angry Birds, to crucial productivity . The smartphone can also double as a portable MP3 player, and a GPS, so it packs a lot of functionality into a pocket-sized gadget.

Role: The smartphone works when you still don't need to be cranking out reports or spreadsheets--but you want to be able to if it becomes absolutely necessary, or if you want to be able make and receive phone calls, but you also want a mobile gadget that can fill multiple roles (like music player and GPS) so you can carry fewer devices.

eReader. The eReader barely made it onto this list, but it fills a niche role worth noting. If you have a device like a Kindle or a Nook, you can carry with you a veritable library. I can't imagine going on any trip--business or pleasure--where having hundreds of books on a light, thin gadget wouldn't be plenty of reading material. From a business perspective, the ereader can also be used and other formats--depending on the device--so you could carry white papers, case studies, quarterly reports, or other business documents. The ereader can typically store and play music as well, so you may not need the MP3 player, and some ereaders--like the Kindle and Nook--are also capable of at least some Web browsing. The Nook Color, in fact, can run Android apps so it blurs the line some with tablets.

Role: The ereader is definitely , but whether you want to read the latest Dan Brown novel, or you need to review a lengthy project status report, the ereader is a much better platform for reading than the smartphone. However, the ereader is even less functional than the smartphone when it comes to productive potential.

Tablet. The tablet is part smartphone with thyroid problem minus the phone capability (for most tablets, at least), and part without a physical keyboard. It is less portable than the smartphone because it is typically too large to fit in your pocket, yet it is much more portable than netbooks or notebooks because it is slim and light--like carrying a hefty magazine. Most of the tablets today run a version of a mobile OS like a smartphone--iOS, Android, WebOS, etc.--so they may not be productivity workhorses. But, the tablet is capable of as netbooks or notebooks, while also doubling as a portable entertainment center with movies, music, and books.