Bascially, there are 8 bits in 1 Byte!!
Syntax and case is very very important and should not be taken likely, as you could end up buying a drive which is 8 times smaller than you originally intented!!! How could that happen???
Well 8Gb is actually 8Gigabits [Remember: 8bits in 1byte], so in reality 8Gb[Gigabits] is 1GB[GigaBytes].
A little table may explain it better;
GB/GigaBytes Gb/Gigabits Bytes Bits
1 8 1073741824 8589934592
A common scam is to list a drive as 1Gb, most people don't realise the difference so the think their getting a one GigaByte drive, in actual fact the drive is 8 time smaller [128MB]. NOTE: 8GB drives are uncommon and retail at £200+, so if you're getting it too cheap it's probably isn't what you're expecting!!
For reference, in order of size;
1 Kilobyte [KB]
1 MegaByte [MB]
1 GigaByte [GB]
The biggest problem is that within the industry it common for GB and Gb to be interchanged and mean the same thing ie. GigaByte but strictly speaking this is not correct!! And this is why people get caught out. Finally, when you buy a stand disk drive from Seagate/Western/Hitachi/Fujitsu.. etc for your PC you will always be quoted in GigaBytes [GB].
Hopefully this helps, and will avoid some people getting burned by basic syntax and techno gobbley gook!!