Nectar and juice are two different things, although both are liquid substances. Nectar is a sweet liquid produced by plants, especially by flowers to attract pollinating agents such as birds and insects. On the other hand, the juice is also a liquid gotten from a plant, especially from fruits and vegetables.
Although this liquid is present in its natural state inside the fruit, however, it can only be gotten by squeezing or mashing fruits. In contrast, nectar is naturally available and does not require any further process before it can be gotten. In terms of quantity, the juice is present in fruits more than the quantity of nectar secreted by flowers.
While juice is mostly taken by human beings, nectar is a sweet liquid for animals, most especially birds. Although nectar can be taken by humans too, it is not as common as how people take the juice. In terms of thickness, the juice is not as thick as nectar.