Let's look at rotary laser level basics, as well as the more price-friendly products on the marketplace.
What is a laser level?A laser level is fixed in one place, often on the wall itself or at some central point in the room. A continuous laser beam forms a solid red line at a level point on horizontal and vertical areas. Fixed-place laser levels can be purchased for as little as $15. Rotary lasers tend to be more expensive.
Does the laser level itself need to be on a level surface?No. But it does need to be reasonably within level.
What are preferred applications for laser levels?Bubble levels are terrible at indicating long lines (i.e., beyond the length of the level itself). So, any instance where you need to determine a line more than 3-4 feet is perfect for laser levels.
Laser levels work well for showing long lines (that don't necessarily need to be level), such as a line of the floor for tiling.
What is a rotary laser level?Fixed laser levels throw a level beam against a wall or floor.
By contrast, rotary laser levels sit in the center of a room--preferably on a tripod--and cast a laser beam in a complete 360 degree circle. Thus, the rotary laser can show a laser "chalkline" on all four walls with complete accuracy. Or it can cut a vertical swath, hitting walls, ceiling, and floor simultaneously.
Shown: Johnson Acculine Pro 40-6050, an entry-level rotary laser level. This image shows how the laser beam can rotate to cast a line on walls.