Why have I paid for 8gb but only got 6?

I've recently bought an iPhone 4 with 8gb but when I synced it on iTunes it says I only actually have 6gb. This is with "other" which I know how to reduce. Also on my iPhone itself on settings it says I only have 6gb. This memory is very valuable because I only have 8gb and if I'm honest I think I should have bought at least 16gb. Can you help me by answering with an easy answer baring in mind I'm only 15 and you'll probably confuse me?

Some of the gb are already used since some apps are automatically on your phone when you first get it.

Those other 2gb are used up by the operating system, you can't do anything to make that memory usable. If you bought the 16gb, you'd only be able to use about 14gb of it.

It's a little bit unclear from Apple to be fair. They say you're buying 8gb, but a couple is taken up by the IOS etc. It's annoying really.

The Iphone operating system uses that memory. The 2gb is used to run your phone. My regular old android uses 1.5 gb of my 8gb too.

This is normal. 2 GB is used by the OS and the rest is for you. Also its also in hard drives in flash drives. Part of the space is used for the driver software so the device could be used.