Believix is the fourth transformation earned by the Winx. It gives them the power to make people believe in fairies and magic. This was earned by the Winx in Season 4. It is not the final form, there are more fairy forms other than Believix. Enchantix is only the form that a fairy needs to become a full-fledge fairy.
In order for a fairy to earn this form, she must make someone to believe in fairies and magic, and can only be earned in Earth or somewhere people don't believe in magic.
It consists a mini-skirt or caprices, gloves or braclets, and knee-high socks or ankle-boots. Their appearance looks more casual and like Earth clothing.