Needs refer to the basic biological needs of human beings like need for air to live, need for water to drink, need for shelter to live, need for safety etc..
When these needs are backed up by something, it becomes demand. For example you have a need to drink, Now what to drink? If you want to drink water, it becomes your want for water.If you want to drink milk, it becomes your want to drink milk.
Similarly when there are wants and the people are willing to fulfill that want and they are able to fulfill those wants(Like they have enough funds to purchase water or milk) it is known as demand.