[Kuuipo]

Powder sets the foundation, mattifies the foundation, makes it last longer. There are so many powders! Some add a bit of sparkle, some clarify-cut oiliness, some have polyymers that prevent them from settling in wrinkles,some are talc or cornstartch based or mixtures of these or synthetics (which is not always a bad thing). Powder should not be used to hide blemishes but to finish your makeup-think "airbrushing". Some people like to match their powders exactly to their foundations for more coverage, some choose a transparent powder-its the preference of the person and how much cover they need.
You will want to apply powder with a clean puff. I like disposable cotten pads-and buff off the excess with a buffing brush-although some people fluff it off with a powder brush for less coverage. I am a big powder person so I press it in, wait, buff it down and repeat. The result is matte skin but a natural no makeup look (works well if you get photographed alot).

Powder in compacts are more concentrated,heavier.

I am not a compact care. I carry oil blotting tissues impregnated with transparent powder.