Majority of manufacturers who use Android on their devices load it up with their own custom skin atop the default Android interface.
Samsung, a major player in the Android race loads Touchwiz on their devices. Other major interfaces by manufacturers are Sense 4.0 by HTC, Motoblur by Motorola and Timescape by Sony.
They serve as some sort of differentiator that you could use to identify devices made by that particular manufacturer. This is supposed to be something good right? – sometimes not.
That a custom skin be loaded atop the default Android skin is supposed to be something that enhances the experience and give you certain features that do not come bundled with the default skin. This is demonstrated in new features that Samsung have implemented in both the Galaxy SIII and the Galaxy Note 2 such as pop-up play.
Sense UI from HTC for example is a memory hog. In the latest iteration (Sense 4.0), efforts were made to minimize the effect on memory but still there have been complains about the One X killing background processes.
Touchwiz from Samsung has also been the cause for many update delays in the past and currently. The original Galaxy S was denied update to Android 4.0 Ice Cream Sandwich because according to Samsung Touchwiz could not co-exist with the installation. Examining the reason by Samsung, it was denied ICS not because it could not run ICS but because it could not run “Touchwiz”.
Sony too has been guilty of denying flagships update back when they were still Sony Ericsson. The gaming centric mobile phone; Xperia Play is stuck in Gingerbread land with no hope of ICS or Jellybean.
Something to note is that the more feature packed a manufacturer skin is, the longer it might take to update it to the latest Android version.
To avoid all custom skins, the right thing for you to do is to purchase a Nexus device usually made by a manufacturer in conjunction with Google. With a Nexus device you do not need to worry about third party skins as it ships with the default Android skin.
The problem with Nexus devices though is that they may not fit your specific taste. They stand as a reference to the direction that Google wants to take with Android.
In the end it is all about choice. Samsung tops the Android league, HTC also manufacturers some stunning devices such as the One X/S. Motorola too is worth a mention with the announcement of new devices.
What this means is that the openness of Android provides you with many choices. There are quad core devices, dual core devices, rugged devices and so on. If you want pure Android experience however a Nexus device is probably the way to go.
Before buying an Android device from a manufacturer, you should do research and find out how they handle OS updates. It is tempting to buy a device just by being intrigued by the extra features it packs but research will save you the stress of buying a device that will not get updated.