its HEX
so a-f and 0-9
8 sets off 4s

so 0000:0000:0000:0000:0000:0000:0000:0000 - ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff

it can also be shorten down. like 0000 is written like this ::

so 0000:0000:0000:0000:0000:0000:0000:0000 is realy :: in short format

to put that in on a real ip context
2001:470:dc100::100
is short for 2001:0470:dc100:0000:0000:0000:0000:0100

so a zero at a start can be removed withouut adding a :
a full sets off 0 in a line 00000 is ::
:: can only be used onces so ::: is invalid

if you want to test ipv6 you can get a free tunnel from http://tunnelbroker.net/

(IPv6 in hex gives enough ips to every corn off sand ine the world, had it been a-z it would be every atom in the universe)