Mechanical bosses

Mechanical Boss is a term which refers to any of the three Hardmode bosses that can be summoned by "Mechanical" items which can be crafted and can drop from any hardmode monster (except for statue spawned enemies and meteor heads) with a 1/2500 chance until the respective boss is defeated. These items are listed below. Besides being summoned, there is a 10% chance that one will be spawned each night after the first Demon/Crimson Altar is destroyed. They will continue to spawn randomly until each individual one is defeated at least once, the only condition being destruction of a single Altar.

Once any Mechanical Boss has been defeated for the first time:

After all three Mechanical bosses have been slain (Only one needed in the Console/Mobile versions), the message "The jungle grows restless" will appear, and Plantera's Bulbs will begin to grow in the Underground Jungle. In the Desktop version is it possible to kill the mechanical bosses prior to killing the Wall of Flesh, but the Wall's defeat is still be required to unlock Plantera.
 * The Steampunker will arrive if there is a house available.
 * The Truffle will begin selling the Mushroom Spear and Hammush.
 * Solar Eclipses can occur.
 * Red Devils and Lava Bats will spawn in the Underworld.
 * Yelets can be dropped by any enemy in a Jungle biome and Underground corruption/hallow.
 * Life Fruit will begin to grow in the Underground Jungle.

Achievements
Buckets of Bolts is the achievement you earn for defeating The Destroyer, The Twins, and Skeletron Prime for the first time.

Mecha Mayhem is the achievement you earn for defeating the three Mechanical Bosses in one battle together.

Tips

 * If you aren't confident in your ability to defeat the bosses, but you want access to the hardmode metals, you can refrain from destroying any Altars until you are ready and instead get the ores/bars from fishing up crates. Crates may yield hardmode metals once hardmode is reached.
 * In the same line of thought, you may want to refrain from opening crates until hardmode, as the hardmode ores are more valuable.
 * Another option is to flee underground or log out and back in again once one of the messages appears. This prevents the boss from spawning, though they can still be manually summoned underground.