Hi janlo.
ONCE is the acronym for Organización Nacional de Ciegos Españoles (National Organization of Blind People).
Their loto is more or less as any other: You buy tickets with 5 digit numbers. There is a different loto every day.
The ONCE loto serves blind people (and other people as deaf persons, psychic disminished,...) in two main ways: It gives them a decent payed job (selling loto tickets) and as a financial source for deploying many projects (removing architectural barriers, investigating new technologies to make their lives easier,...).
It is the only legal private loto existent in Spain, and it is quite popular. Not only you can get rich, but you are helping people with your money.
Fernando