Foundations range from sheer coverage to full coverage, and are skin-colored products designed to even out skin tone and cover minor problem areas. Foundation is designed to be used over the entire face, blending up into the hairline and ears, and down over the jaw-line (to prevent a mask-like effect). Some foundations have diffferent finishes, depending on the appearance you prefer. Some are matte, some are more natural, and some are shimmery/dewy. Foundation is applied in different ways, depending on the formula. Most common are foundation brushes, cosmetic sponge wedges, and fingers. Again, depending on formula, the application techniques vary, but most are rubbed or blended over the skin. Foundations are generally set with powder, to make them wear better.
Concealer is a more heavily pigmented product similar to foundation. Often found in three forms- liquid (either in a squeeze tube or wand applicator tube like lip-gloss), stick, or creme (usually in a pot). Concealor is a heavier product than foundation, though liquid concealers can be fairly lightweight. Concealer is used just as it's name indicates- to conceal. People generally use these to hide birthmarks, blemishes, strong discolorations, and dark under-eyes. Concealers often come in corrective colors as well, not just skin tones. Mint green is used to counteract redness, lavender is used to counteract sallowness (yellowy spots), pink helps brighted the eye area and is often used to correct a greenish tinge which you can get from illness, and peach or yellow concealers are used to counteract dark bluish areas. Concealers are applied with a patting motion, not a rubbing motion. This helps build the coverage in light layers, without irritating the skin (or damaging delicate eye area skin). Concealer can be used alone (set with powder so it doesn't melt off your face) or along with foundation or tinted moisturizer.
Tinted moisturizer is a cross between your regular facial moisturizer, and foundation. It imparts a small amount of color- just enough to even out the skin tone somewhat, without much coverage. It is used in a similar manner as foundation. Some consider tinted moisturizer another form of foundation.
Powder comes in a few forms. Generally, powders are sheer to translucent. They may impart a slight color, but generally not enough to give any coverage or even out skin tone. Powder is generally used to set foundation and concealer, and to tone down overly dewy looks, or to combat oiliness. Powder comes in two forms- pressed and loose. Pressed is either applied with a puff or a brush, and is easier to transport as it comes in a little compact with a mirror and puff. Pressed doesn't give as flawless of a finish as loose however. Loose comes in a container with a sifter top, and is applied with a brush. Loose powders give you more control over the amount of product used.
Does that help? I can explain anything you are still confused about in more detail if you need.
