Tool

Tools are items used by the player while held to perform actions faster and more efficiently, to gather materials not obtainable by hand, to gain information, or to perform completely new actions. With the exception of the clock and compass, tools do not stack in the inventory.

Diamond tools vs. Gold tools
Gold tools mine blocks the fastest of all tools in the game and have the highest enchantability (making it possible to get Silk Touch with a level 1 enchantment, which is impossible with another tool tier), however they also have the fewest uses (33) and cannot harvest most ores. Most players with the resources opt instead to use diamond tools, which are nearly as fast and have 1562 uses before breaking.

Trivia

 * If you switch from a more valuable to a less valuable tool, without releasing the mouse button, just before a block breaks, the time required will be similar to using the former tool, while the mined block will be counted against the latter's durability. This is true for shovels, axes and pickaxes. However, in the case of pickaxes, this only works if the latter can mine this type of block, e.g. for iron and stone pickaxe while mining iron ore it would work, but for diamond and iron pickaxe while mining obsidian it wouldn't. Profitable uses for this (in terms of time saved) are rare.
 * Jens told us around 1:30 in this video that it is going to be possible to upgrade your tools, which is now known as Enchanting.