Items are placed in categories based on the version/release of Minecraft PC they were released in.
Minecraft 1.0 was the first Minecraft version to be released without an alpha or beta status.
Minecraft 1.1 was a big update. Many new items (like spawn eggs) were added, along with the superflat world generation setting.
Minecraft 1.2.1, like the Minecraft 1.1 update, added a lot of new items. This update also introduced the jungle biome to the game!
Wood planks and sandstone blocks arrived with the Minecraft 1.2.4 update.
More items were added in Minecraft 1.3.1, yay!
Minecraft 1.4.2 was dubbed the Pretty Scary Update.. In case it was not implied by the name, lots of spooky items were added.
Along with the Minecraft 1.4.6 update came fireworks and enchanted books.
Minecraft 1.5 was named the Redstone Update.. Lots of redstone items were added!
Minecraft 1.6.1 was named the Horse Update. It not only introduced horses, but items like leads and carpets!
Minecraft 1.7.2 introduced new fish and a lot of other new items. Other features added in this update include new biomes, structures and commands!
Minecraft 1.8 not only changed PVP, but lots of new items were added.
Minecraft 1.9 graced us with many new items including beetroot. It also introduced the controversial new combat system.
In Minecraft 1.10 new mobs were added.. Which means new spawn eggs were added! Unfortunately, new spawn eggs were the only items introduced in this update.
Minecraft 1.11 was named the Exploration Update.. It added lots of new items, including spawn eggs and the shulker shell.
The knowledge book was introduced in Minecraft 1.12, along with parrot spawn eggs.
Minecraft 1.12.2 added a new logo to the menu screen. No new items were introduced in this update.