particle physics - Differences between reconstruction- and generation-level variables in HEP data

I am working on a CMS - related project where the ROOT trees contain both reconstruction-level and generation-level particle variables (like mass). However, I don't know the basic difference between the two or when we prefer using one type of variable over the other?

Answer

Real data goes $$\text{Collisions} \to \text{Detector} \to \text{Trigger} \to \text{Reconstruction}$$

Simulated data goes $$\text{Event generator} \to \text{Simulated detector} \to \text{Trigger} \to \text{Reconstruction}$$

Generator-level is the information from the first step, before the events are passed through the simulated detector. This is also sometimes called "MC Truth" or "Truth level".

Reconstruction-level is the information from the last step, after the reconstruction algorithms have been run over the data from the simulated detector.

You can compare the two in order to quantify things like efficiencies and resolutions.