In Minecraft 1.8, numeric IDs will no longer exist. If you hit F3+H, you will see more information about the thing you are hovering over in the GUI. FOr example, if you hover over diamonds, it will say underneath "minecraft:diamond". There is a problem, though, it still says:
Diamonds(# 0264)
minecraft:diamond
Why is the numeric and non-existent ID still there?!
MC-59426
Numeric IDs Are Atill Shown When F3+H is Hit
Resolved
Works As Intended
0
(Unassigned)
Unconfirmed
Minecraft 14w21b, Minecraft 14w25b, Minecraft 14w26b, Minecraft 14w26c
Retrieved
Because numeric IDs are still there. If you transfer an older map over to this version, it still needs the IDs to know which block is which.