Laser 360 range sensor from vacuum cleaner Neato XV-11 is one of the most popular and cheapest way to get lidar for robotics experiments.
Sensor has two connectors - first is an ordinary serial port, another is power for motor. Despite on XV-11 Lidar’s price, it provides pretty good measurement data with transmission rate up to 5Hz. Of course there are drivers for ROS, which consume raw sensor data and convert it LaserScan format.
LaserScan is an array of range sensor data withing some angle range. Neato Lidar provides range data in 360 degrees. LaserScan information can be used in robotics for SLAM - map building, obstacle avoidance, etc…