A concealer is basically a type of makeup that is used to cover pimples, dark circles and also any other small marks visible on the skin, while a foundation is a skin colored cosmetic that is applied to the face to create a flat, and consistent color to the complexion. You can use both since they are normally used to make skin appear more even in color. Find out more details on http://en.wikipedia.org/wiki/Foundation_(cosmetics) and http://en.wikipedia.org/wiki/Concealer.
Concealer is generally more of a liquid, or something that applies smoothly and should be 1 shade lighter than your natural skin tone. foundation is generally powder and would be applied after concealer in the colour of your natural skin tone. If you have liquid foundation then you dont need to use concealer, but other than that most people use concealer and foundation.
Concealer is for concealing or covering an imperfection as the name implies. It may he dark circles, blemishes, or redness. It is applied to a specific area. Foundation is used to smooth out your skin tone and is applied all over the face. Powder is usually used to set the foundation or concealer so it will last or can be used alone to reduce shine. You can use 1, 2 or all three depending on your needs.
You only should use all three if you need it, like if you have pretty bad acne, discoloration, or whatever. If you do use all of them make sure you don't look cakey!(; concealer- is mainly used for hiding blemishes, discoloration, or dark circles. Foundation- is mainly used to even the complexion to make your skin look flawless. Powder is used to set your makeup so it will last all day and it will also mattify your face to reduce the appearance of oiliness(;