Slat-style beds are a type of bed frame with elevated slats running along the bottom, parallel to and between the headboard and footboard. The mattress rests directly on the slats, so there's no need for a box spring underneath the mattress. Because of the presence of the slats, a separate metal bed frame is also not necessary.
Slat beds have two rails that sit perpendicular to the headboard and footboard, on each side. The slats, often made of sturdy wood, are attached to these rails, one after the other, with a small amount of spacing in between each slat. Because the slats are only as wide as the bed sizes for which they are made, it's relatively easy to replace individual slats if one breaks.
People often find that slatted beds provide equal or better support than a traditional box-spring model. That's particularly the case when the slats are very wide with narrow spacing. Another benefit of slat-style beds is that you only have to buy a mattress and the bed, so you save on the cost of a bed frame and box spring. Lastly, these beds tend to provide more under-bed storage, since the slats are typically positioned higher up than the top of a bed frame sits.
Whereas box springs tend to sag over time, slat-style beds may simply break from heavy weight applied to one localized area of the bed. Thus, slat beds might not be the best choice for a children's room, since children tend to jump on their beds. While individual slats can be replaced, a bed may remain unusable until the replacement, since the portion of the bed with the broken or missing slat will have no support whatsoever.