"mirror neurons" is actually a misnomer: they are neurons associated with a specific (often goal directed) activity - so perhaps "action" neurons, or "intention" neurons, even "idea" neurons!

discovered in the 1980s/90s by Italian neurophysiologists recording single cells in monkey "premotor cortex", neurons specialized for control of hand and mouth actions. Serendipitously, they found cells that fired not only when the monkey picked up a piece of food, but also when the experimenters picked up that piece of food! Cells that respond to an action, seen or done.

action and intention neurons: cells that respond to food picked up to eat, but not picked up to be put away, and visa versa, cells responding to food grasped to be put away, but not food grasped to be eaten.

intention and idea neurons: "… a mirror neuron which fires when the monkey rips a piece of paper would also fire when the monkey sees a person rip paper, or hears paper ripping (without visual cues)."

Humans: evidence from fMRI (and rare single neuron exploration) suggests that humans have analogous neurons, and that they exist all over the brain, not just in the premotor cortex where they were originally seen in monkeys.

source: Wikipedia: "mirror neurons"