The Badniks are the robotic footsoldiers used by Doctor Robotnik in his mad schemes. There have been many different types of Badnik, most of which use an "organic battery" - a living creature imprisoned within it - as a power source. These batteries force Freedom Fighters to hold back in battle, for fear of harming any prisoner. They are primarily given these batteries in Badnik Processing Plants.
The intelligence of Badniks is wildly variant. Some appear to be near-mindless drones that solely work for Robotnik, while others are intelligent enough to show sadism, fear, boredom, rivalry, and humour. Smokey's Badnik actually went AWOL and befriended a Mobian.
For a full list of Badniks, see List of Badniks.
Mega Drive Badniks
From #1 to #100 of the original series (and in the odd later issue), the Badniks seen in the Mega Drive games were the primary type of non-Trooper Badnik. When lacking organic batteries, they could easily capture their own Mobians to use. While intended for specific Zones, types of Badniks would often be deployed in other Zones or massed attacks in order to combat Freedom Fighters.
These Badniks would often show a degree of autonomy, able to make decisions and do things when Robotnik was absent. In Fox on the Run, two Badniks actually took up the habit of trying to eat Mobians, and also showed incompetence at it.
When Robotnik returned as a Drakon Empire proxy, all of these older Badniks were reproduced in their thousands. Some of them were turned into Uberniks, giant versions of their regular selves.
Sonic CD Badniks
These Badniks were extremely short-lived, being dumped in the Palmtree Panic Zone on the Miracle Planet some time after the initial Metallix project failed. Robotnik referred to them as advanced Badniks.
B.A.R.F., on the other hand, was worried about the danger in transporting "wild Badniks" and they were right to be - the Badniks showed hostility and insubordination when transported back to Mobius, and needed to be cowed with violence. They were meant to be part of a plan to outflank Sonic at the Green Hill Zone, but the plan went south when they decided they weren't going to share a Zone with inferior outdated Badniks and obliterated the locals in a violent frenzy. B.A.R.F. was forced to create the Seven Badniks to bring them back under control, and soon sent them back to the Miracle Planet. Sonic swiftly destroyed them all.
These models of Badniks were not seen again, and Robotnik appeared to abandon the Miracle Planet.
As part of his plan to conquer Mobius, Commander Brutus learnt how to make his own Badnik army. These Badniks were a stronger breed to Robotnik's, and possessed a wild variety of garish, ugly forms; rarely did you see two Badniks the same.
They showed signs of intelligence and even arrogance, calling themselves the "new wave" of Badniks. Two exceptions to this were the Badniks specifically designed to imprison Amy Rose and Johnny Lightfoot, which were solely focused on killing Sonic the Hedgehog.
After killing Brutus, Robotnik took control of these Badniks. They were all destroyed in the later electromagnetic pulse.
Flickies' Island Badniks
After being deposed, Robotnik established a new base on Flickies' Island and started a new line of Badniks. This time, they specifically used Flicky birds as organic batteries, and were more powerful than their predecessors; Sonic initially had considerable difficulty dispatching only a single one, and only just managing to do so with all his strength. These Badniks were usually a grey, dark colour and looked much more demonic and menacing than the colourful, cartoony Badniks that usually populated Robotnik's army. Some didn't bear any resemblance to normal Badniks at all.
After this, Robotnik began making more colourful and "traditional" types of Flicky-powered Badniks, which Sonic and the other Freedom Fighters found much easier to destroy. They never showed the same intelligence as the original Badniks.
Following the loss of Flickies' Island as a base, these Badniks ceased to be used.