Memory cards are everywhere. Digital cameras, portable music players, video recorders, voice recorders, and mobile phones. Most devices come with free cards when you purchase them. But not all memory cards (or even memory chips for that matter) are made equal.
Why is memory class important? It depends what your device needs the memory for. Most music players can work with the slowest memory cards since mp3 playback doesn't demand high speed transfer. On digital cameras, specially DSLRs, a slower memory card will take longer to save each photo taken, thus giving you longer time before the photo shows up on your view screen. Video cameras, specially HD recorders, require the fastest cards due to their continuous data stream which has to be saved as quickly as possible otherwise you risk dropped frames or worse, not having the clip saved at all.
Memory card speeds are usually printed on the card itself or on the box they came in and are indicative of the respective MINIMUM transfer speeds the card can handle.
and so on...
But what about on Android phone? Do they make a real-world difference? Yes they do. Even if your phone doesn't require a fast card, it can usually handle faster data. This in turn has a domino effect on the over-all performance of your phone.
This means faster access to data and apps that are saved on the card. The difference between a Class 2 and Class 6 card can speed up start up time, or allow faster multi-tasking. My Xperia originally came with a basic Class 2 microSD card. The more apps I installed, the slower my phone seemed to become. This is due to the bottleneck created by the card to access the card's content. I upgraded to the Sandisk Mobile Ultra microSDHC card which is a Class 6 and has the added benefits of being waterproof and temperature tolerant (Although a side note, most SD cards are quite durable and can withstand quite a bit of physical abuse already)
Are Class 6 (and higher) cards worth the upgrade? Yes they are. The higher the better, although for everyday tasks, anything above class 6 won't make that much of a difference anymore unless you're doing heavy HD video or audio recording. It's a quick and cheap way to upgrade the overall performance of your Android phone (or any gadget that uses a memory card)