Secure Digital or SD cards are widely used in cameras.  Three different kinds of SD cards are used in cameras:

• SD cards (up to 4G),
• SDHC cards (High Capacity 4G to 32G)
• SDXC cards (eXtended Capacity up to 2TG).

SD card speed classifications:

The speed ratings are very confusing for consumers.  The speed Class Rating are the official standard defined by SD associated.  The speed is defined as the minimum transfer speed

• Class 2: 2 MByte/s or 13x
• Class 4: 4 MByte/s or 26x
• Class 6: 6 MByte/s or 40x
• Class 10: 10 MByte/s or 66x

Which SD card brand to get and how much memory?

The reliability of SD/SDHC cards are extremely important for digital SLRs. I learned this lesson in a hard way after my SDHC card  failed and I lost all the photos and videos on the card. Since the price of the SD/SDHC cards are down so much, it is better off to buy good and reliable cards.

Considering the value, reliability and speed, the best cards to use with Canon digital Rebel XS are  SanDisk Ultra SDHC cards. This card is fast enough (15MB per second data transfer), reliable and not very expensive.  I prefer to have a couple of 8G San Disk Ultra cards instead of 16G.