The Inators were plans and devices created by Dr. Heinz Doofenshmirtz as a means of dominating and taking over the Tri-State Area. These items were typically grandiose in scope, involving a complicated solution instead of applying a simpler, more practical method. For example, inventing a "wood magnet" when dynamite would be more effective and faster.
The motivation behind their development was typically a need to exact revenge for an unpleasant incident in Doofenshmirtz's life, or to resolve a situation that annoys him. An example of the latter was the plan to transport Big Ben from London to Danville instead of buying a larger clock because shopping for a clock would be too much of a hassle.
Though the names given to the inventions tended to end in "-inator", so far he had made 23 inventions lacking the "-inator" suffix. It is unknown how he could afford to build all his inventions, though he did mention that he gets a large alimony check every month by his ex-wife, Charlene Doofenshmirtz.
Most of his inators backfired at Phineas and Ferb's work, destroying it or infecting it, leaving Candace without any evidence to bust the boys. One invention called the "Shrink-inator", which he always forgot to unplug, had done away with the boys' work more than once. However, a few were quite successful in nature, such as the time when Doofenshmirtz succeeded in stealing the Key to the City, opening a portal to alternate dimensions, moving the Earth the away from the Sun to an early autumn, and building a Tri-Governor's Mansion on top of City Hall to become Tri-Governor.
When Doofenshmirtz was defeated by Perry the Platypus, it was not always known how he returned to normal, as he would frequently find himself in impossible situations. At one time he significantly aged himself. He had wound up in outer space several times, in another dimension, miniaturized and wrapped up in a small box, stuck inside a whale or betrayed by an assistant/friend.