Pre 2000 powergamers and munchkins were pretty much the same thing; the main difference being that the former were good at creating "twinked out" characters, whilst the latter were bad at it.

The "powergamer" might tell you about the difference between the bladesinging kit and the bladesinging proficiency, and why it pays to combine the latter with fighting with two weapons; he may complain if the game master recognises this abuse and house rules it away.

The "munchkin" will probably tell you about his twentieth level halfling fighter/magician/thief who rides his celestial dragon through the cosmos when not otherwise engaged with the harem of 100,000 young ladies (most of whom are high level ninja-assassins) at his imperial palace situated on his own personal demi-plane.

The "optimiser" is a new term that has come into existence with the increasing presence of an on-line D&D community, and seems to be linked to the proliferation of D20/3e character creation options. Many powergamers and munchkins have labelled themselves optimisers to avoid the stigma of the other two terms. In fact, though, powergamers and optimisers are pretty much the same; both seek to create the most powerful or effective character that they can within defined limits, and the individuals within the groups will vary with regard to their other qualities and aims. Defining one as "bad" and the other as "good" is a false dichotomy.