What is the difference between a full head of foils and highlights?

Highlights & foils are usually the same thing. Highlights maybe done through an old fashioned plastic cap. Foil highlights allow the colourist to use 2 or more different colours, this often gives a more natural finish and the illusion of movement. It means you can place different colours for maximum effect.
A half head of foils can be used to cover regrowth that appears where the hair parts. I would only have 1/2 head between full head. You could alternate between the 2 treatments.

Hello,
A full head of foils and highlights are the same treatment. It depends on how you would like to achieve the look you want, but if you normally tie up your hair or for any hair-ups, you can style your hair comfortably with a Full Head of High Lights (Foils). A full head is all over your head and half head will be spread over the crown and side, also usually the top layer of hair. If you hair is a bob or shorter, a half head might be enough for you.

Hi! Highlights and foils are basically the same thing. However you can use the cap or other techniques to create highlights but foils are the most popular and, in my opinion, most effective and creative method.

Foils is a method use in highlighting hair, there are other methods such as the cap method. Different people use different methods to highlight hair.
Getting a full head or half head highlight depends on the style you want.
I will advice you ask your stylist for a hair highlight catalogue so you can choose the style you like. Choosing the style you want will help your stylist to know if you need a half or a full head highlight.
I hope i have answered your question, should there be any other thing you want to know about hair treatment you are welcome to ask.