You need to be more specific about what you want to build.
A true LIDAR system infers the round trip time of photons, either directly (time-of-flight) or based on some proxy like phase or frequency shift (AMCW/FMCW) LIDAR.
Lots of people include triangulation systems, which I disagree with. For example the RPLidar A2 is just a nicely packaged version of Kurt Konolige (et al)'s RevoLDS [1]. It projects a laser spot and takes a picture of it. Add in some known geometry, and you can use triangulation to measure distances. It's easy to build, you can do it with any old camera and a laser pointer. It's incredible how overpriced the A2 is, given that the Revo was published as $30 system.
A true LIDAR is much more difficult to build. If you go time-of-flight then you need a good laser and excellent timing electronics. If you go phase-shift then you need beam mixing/separation optics, beam modulation and a phase detector. Neither of these are easy for the average hobbyist. If you're interested, buy a laser tape measure from Leica and have a look inside to see how it works.
I belive the LIDAR-Lite is a proper LIDAR system - it ranges up to 40m which is a good hint that it's probably a true time of flight system. The Scanse Sweep is a LIDAR-Lite turned into a 2D scanner, like the sibling post. A good indicator is price. Real LIDAR systems are expensive - starting at £1k typically for something like a Hokuyo. They also tend to be much better built than the hobbyist stuff.
In principle all you need is a pulsed laser, a big collecting lense, a fast rise-time photodiode and a good (picosecond accurate) timing circuit.
