The high capacity microSD, aka microSDHC, are those microSD cards with capacities in excess of 2 gigabytes that adhere to the SD Association's standard for high capacity cards. What does the Class Rating mean? What does that "C" with a 4, 6, or 2 inside it signify? 

The class refers to the data transmission speed (DTS) of the card, its ability to accept and send information from the device in which it is installed. If you know how quickly a device can read and write to the card you can more closely match the card to it. There is no sense in putting a Class 6 card in a device that writes so slowly that a Class 2 card is sitting there twiddling its virtual thumbs waiting for bits.

As a rule of thumb the class of a card is the sustained data transfer speed (DTS) in Megabytes per second (MBS)

• CLASS 2 = 2 Megabytes per Second

• CLASS 4 = 4 Megabytes per Second

• CLASS 6 = 6 Megabytes per Second

A good rule of thumb is to install the matching or next fastest class card in your device to get the maximum performance at the least cost.

If you have a device that is capable of fully utilising a CLASS 6 card and you saddle up a CLASS 2 you will impair its functionality. In a camera that will slow down how quickly you can take the next photo. In a device that is capable of transferring 1.5MBS if you install a CLASS 6 card you are wasting your hard earned money. Prices for a CLASS 2 and a CLASS 6 device can be as much as 100% premium for the CLASS 6 over the CLASS 2 for the same amount of storage.

Class ratings also apply to the full sized SD cards.

CLASS 2 SDHC Card               CLASS 2 icon

    
CLASS 4 microSDHC      CLASS 4 icon up close and personal

  
CLASS 6 SD card            CLASS 6 Icon

The images are from Kingston products. The CLASS icons are not exclusive to any particular manufacturer but are used across the industry and are trademarks of the SD Association.  As can be seen the symbol is a san-serif C with the numeral of the class rating inside.

When shopping on eBay for SDHC or microSDHC know your class. If a seller has obscured the class icon or does not let you know from the description what class he or she is selling either ask or move on.