Simulation method issues

HI guys,

I am a new to the PX4. I am confused about the simulation.
Can anyone give a brief introduction to SITL、Gazebo、HITL、MAVROS、DROONEKIT? And what are the differnece between them?

I want to do a project about collision avoidance of PX4 with a 360 Lidar. Which simulation method I can use?