A foundarion primer is to allow the foundation to stay on longer and to glide on your face easier and just to generally make it appear better. If you have oily skin you can get a primer specifically for oily skin, if you can see lots of pores on your skin you can buy a primer to conceal and tighten pores, if you have redness; you can again get a primer specific for that to tone down the redness.. It is a good product but not essential.
A highlighter reflects light from your skin which could be a good way to subtly conceal imperfections. It gives your face a glow and makes it look more refreshed. The most common places to put highlighter on is on the cheekbones, browbone, cupid something lol ( just above lips lol). I recommend the touche eclat ;)
makeup primer or face primer- it makes your makeup last longer through out the day depends on what brand you use.
highlighter- they usually make highlighters for like your cheeks such as the mineralize skinfinish natural from mac makeup they have one or you can use like white sparkles i guess and put this on the tops of your cheek bones and where ever the sun shine on your face you put the highlighter makeup there.
Makeup primer is what you put on before your makeup. There's foundation primer, eyeshadow primer, and lip primer. They all make the makeup stay on longer and look better. Highlighter is a product used to highlight certain points on your face and give that 'glowing' appearance.
Primer lets your make-up stickk better and last longer. A highlighter in makeup is a lighter colour that adds highlights to your skin to define your face more. If it were eyeshadow, it would be a light, whitish or shimmery eyeshadow.