A test shoot is an opportunity for the team of photographer, stylist, model, make up and hair to produce pictures to put on their portfolios, websites, etc..
It's is generally not paid (in London and Milan from my experience, I've heard that in NY is different..) but a great chance to work with new people, to experiment and being creative.
In London you have this very good website called WHOISTESTING.COM where you only have to put an example of your work and people can contact you to collaborate.
Here some pictures from the backstage of a test shoot I did recently:
LOVELY MODEL POSING FOR BACKSTAGE SHOT
THE FINAL PICTURES
PHOTOGRAPHY BY ALICE ROSATI
STYLING BY IRENE DE SANTIS
MAKE UP AND HAIR BY CHIARA GUIZZETTI