Advice on choosing the right SD card?

I have just got the Nikon D3100, and I need a SD card. The problem is, I don't know how to compare them. What are writing and reading speeds for cameras? How will this affect the performance of the camera and usage of the memory? Also, how will the class of the SD card make a difference?
Many Thanks

Added (1). *WHAT are writing/reading and in reference to the classes Nikon has recommended a minimum of a class 6 for my camera

For shooting stills, a couple of 4 gb Class 4 SDHC cards are what you need.

If you are going to be shooting video, you need Class 10 SDHC cards

These days, the read/write speeds of the memory card should not effect the performance of an average DSLR. The speed bottleneck will be the camera itself.

Avoid cheap knockoffs. Go with the largest capacity disc you can that will fit your camera.

Stop overthinking this, get a class 6 card, brand doesn't matter, get an 8gb card.

Do as the Makers of the camera suggest and get Class 6 cards, there's no point in getting Class 10 or the new UHS Class 1 cards as the camera is too slow to make use of the extra speed. Though if you used a fast card reader downloading to your computer would be faster.

I always use Sand Disc Extreme cards, all cards are not created equal, the San Disc can repair itself from corruption as it can re-write the control chip, and another page of memory would be used if it detects a corruption during POST (Power on Self Test) when you power up your camera. Cheaper cards can't do this. They are not hermetically sealed and waterproof as Extreme cards are and just not as rugged, you get what you pay for with cards.