illuminator is a face makeup product used to add a hint of shine and shimmer to your face to give your skin a healthy glow and to brighten the entire complexion,They usually covers your whole face giving you a sheer dewy look.Sometimes it has sparkles or glitters in it but very sheer. Illuminator comes in a liquid.cream and powdery form as well. The liquids and creams are best for dry skin and powder illuminators are best for oily skin types.One should be very careful while applying because wearing too much illuminator will make your face look oily instead  of shimmery.Mostly illuminators are used by mixing them with moisturizer or liquid foundations.I recommend  them to use as a night time look because during the day you will look too sparkly and noticeable,it can be worn alone and it does not conceal.

A highlighter is lighter than foundation or skin and can be a powderliquid or cream. A highlighter comes in a few colour but always option for pinky peachy and silver colour.Highlighters let you reflect light and highlight the features of your face by lending some shine to it that is most flattering to you, but you need to apply and blend it well.These are the main areas where you apply highlighter
1-top of cheekbones.
2-middle of your head.
3-down to the end of your nose.
4-your cupid bows.
5-above your eyebrows.
6-on the chin.
it gives much stronger effect than illuminator

source by maha's beauty blog