A sunroom refers to any room with very large or numerous windows through which sunlight can enter and provide an ample view of the outdoors, while the room itself remains fully enclosed. A sunroom may even have one or two walls made entirely of glass, though this is not necessary for an area to constitute a sunroom. An enclosed porch, also called a sunporch, is a type of sunroom.
Since a sunroom constitutes any room with extensive glass along the walls or ceiling, a solarium is a type of sunroom. The term "solarium," though, refers to a very specific type of sunroom. A typical solarium contains no drywall whatsoever, but consists only of glass wall and ceiling panels, held together with thin frames that stretch across the walls and ceiling. A solarium may also be referred to as a conservatory.
While the solarium itself consists entirely of glass and framing, the room need not be completely surrounded by glass to constitute a solarium. A solarium is typically built as part of the house, whether as an addition or as part of the original architecture. Quite often, the solarium will have only three walls, with the main house itself providing the fourth wall. Additionally, some solaria contain no ceiling.
Solaria and sunrooms both allow you to bask in the sun or just enjoy the beauty of the outdoors, but from the comfort of the indoors. They also provide a great environment for growing indoor plants. The choice between a sunroom and a solarium is largely an aesthetic decision, as each structure presents a different look and style. If you want as much sunlight as possible, though, a solarium will provide you with optimum exposure.