A casa particular is a bed and breakfast (room and board) in a private home in Cuba. Private citizens are allowed (meeting certain conditions and paying the required tax) to rent out rooms to tourists.
They can offer lodging and meals. The Casa particular offer an economical way to see Cuba and provide a quality service while allowing you to get in close contact with the Cuban people.
An experience that can't be matched by state run hotels that separate rather than unite Cubans and tourists.