Most of the MIDs floating around out there run the Android operating system, an OS developed initially by Android Inc., but continued by Google after their acquisition of the company in 2005. Google developed Android along with the Open Handset Alliance, a collection of 80 or so hardware manufacturers, software companies and other related entities. Android is free and open source, meaning that anyone can gain access to the source code, making development for it much easier than for a closed operating system like Apple iOS or Microsoft Windows. New versions and updates come out regularly, and new apps even more frequently. The OS is maintained and updated by the Android Open Source Project.
There are many versions of Android (all, incidentally, named for foods), up to the most recent release: Android 4.1 Jelly Bean. Early releases were optimized for the smaller smartphone form factors, but Android 3.0 and beyond were made to more easily handle scaling of applications to the larger tablet screen sizes. Unfortunately, the vast majority of MIDs floating around today (especially the cheaper ones) are running Android 2.2 or 2.3 versions. Many of the newer and more expensive devices, however, are running the later and more secure versions of Android, 3.0 and beyond, and even some of the incredibly cheap tablets that are starting to emerge are running 4.0 Ice Cream Sandwich.
Aside from the standard Android installations that are available, many manufacturers add their own software to create an interface that differs somewhat from vanilla versions of the Android OS. Some keep it mostly the same, but add useful utilities, while others make the operating system nearly unrecognizable (which is made possible by the extreme customizability of open source code). Often, the cheaper tablets do not allow upgrading of the Android operating system, leaving you stuck with whatever version was on there when you bought the device (creating security and newer software compatibility issues). Devices certified by Google are able to include Gmail, the Chrome browser and other similar apps that have become standards, and allow access to Google Play.
Formerly known as the Android Market, Google Play is a central repository of Android apps for download or purchase. It includes a wide variety of games, productivity apps and the like. When it was rebranded as Google Play, movies, music and other media were added to the mix (perhaps to compete better with Apple iTunes). Unfortunately, not all devices have access to the market. Many devices direct you to their own app stores, or even potentially less secure third-party markets. For the cheaper devices, this is often Archos's AppsLib market, but some manufacturers come up with their own. The cheap MID M80003W, which runs Android 2.2 Froyo, directs you to its own very small market. Some higher end manufacturers have their own, as well, but in those cases, it may actually increase security by allowing them to approve the apps you can download, as there is little vetting of the apps in the Android Market. Amazon even created its own Appstore for Android that many devices can access.
Despite all this Android talk, there are also MIDs floating around with other operating systems, including some devices with the Intel x86 compatible Atom processors running both a mobile version of Windows and Android simultaneously. These are harder to come by, and not very cheap, but Intel and Microsoft are trying to gain a foothold in the mobile market. iPad, the definite leader in the current tablet market, runs Apple's proprietary iOS operating system, but no other device can avail itself of this OS. Though Apple has the largest market share, the various Android devices are collectively their biggest competitors.
Read on to find out what you can do with these powerful mobile devices.