A collision is an event in which two or more bodies exert forces on each other for a relatively short time. For computational physics (as video games and physical simulations), collision effects are reproduced with the use of Collision Detection systems, which determines whether two objects have collided or not, and after that, Collision Response acts, dealing with simulating what happens when a collision is detected.
Most mesh files have a collision data linked to them, which makes an object/scenery impenetrable (solid) and detectable by the game's collision detection system. Otherwise, if a collision file is not present, it will act like a ghost (penetrable/not solid).
Collision files usually have the same name of the related mesh. For instance, NHood1.w and NHood1.ncp, or Ball.m and Ball.hul.
There are two formats of files used for collision data: hul and ncp. A third file currently unused is the COL, which is still present in some levels' folder.
HUL files are basic, containing a set of vertices, edges, and planes to create the actual hull, and then a set of spheres that fills the interior of the Hull. More detailed breakdown of hul files' structures can be found at Ali's website. 
There are five known tools related with hul files.
- The GUI editor HULEdit or the original script based Hul2txt and Txt2Hul, used for hul editing.
- Srmalloy's HULScale, used to morph/scale a hul file.
- A POVRay script called Hul2Pov, used to view a hul file.
- Prm2Hul. A tool created by Jigebren that enable the automatic creation of a hul through a prm file.
- Jig's Blender Import-export Plugin. Enable hul importing and exporting in Blender. This is the most used method nowadays.
NCP is the file used by Re-Volt for setting collision data to instances, tracks and some Stock Models. Unlike hul files, this file is used by rigid objects which a RC car is not able move in real life, like walls and etc.
Ways for generating or editing ncp files are the follows:
- Using PRM2NCP, a drag and drop tool that comes with the game inside its instalation folder.
- I2PRM, works as the same way of PRM2NCP.
- Asetools. NCP files are generated automatically when converting ASE files.
- Jig's Blender Import-export Plugin. Enable NCP importing and exporting in Blender.