Concealer and foundation are both used on the face, but concealer tends to be thicker in consistency to cover problem areas such as dark under eye circles or pimples. In my makeup stash I have several different colors of concealer for different purposes. For instance, I wear a lighter shade of concealer under my eyes because I have dark circles. I put it on in the shape of a triangle starting at the bottom lash inner corner, go down towards the tip of my nose, then up & out towards the outer corner of my eye. I use this shade as a dark circle corrector and a highlighter.

Then I use a green concealer to cover any red acne pimples (green cancels out the redness). I also have a concealer the same shade as my skin color to put on my lips. Because my lips are very pigmented, the concealer serves as a base for lipstick and helps get the true color of lipstick. Some people also use concealer to use as an eyeshadow base, but this just causes creases in my opinion. Some of the brands I use are Covergirl, MAC cosmetics and Almay. In my opinion, the Covergirl gives the best coverage for the value. Hope this helps :)  

Juicy provided some great info and tips, I hope you found them helpful! There are also some articles around the site that you might want to read. In addition to concealer and foundation, the best way to keep you makeup in place is to use one of the best makeup primers available to you. Once you've figured out what primer is best for you, check out these makeup tips with products that hide skin problems. If you don't have any major skin issues, but want some makeup advice, take this quiz to find out what foundation is right for you. Good luck!