Simulation is a useful scientific technique; hence it employs and incorporates all the methods of science in presence or absence of experimental data.
As scientists we are concerned with studying many different kinds of Physical objects and systems. These systems vary in their scale and complexity. To give examples, one may be concerned about studying a simple hydrogen atom, which is placed in a surrounding magnetic field. One may be interested in shooting a beam of electrons or neutrons to hit a target material and study the interaction of the beam with such materials and study the beam dynamics.
Another example, which is above this scale in size, is a molecular system such as the motor neurons. One goes above and sees how a complex object such as the arm of a robot or that of an animal operates and exerts force on a target. If one goes still above in scale to a higher size, one can study a planetary system or a cosmological system.
In particle accelerators in which I have worked or similar such facilities around the world one is interested to study the behavior of elementary particles and their decay and interaction. All these systems can be observed with experimental set ups, eg a particle accelerator is a facility that accelerates a specified beam of very tiny (elementary) particles to a desired energy and lets them meet at a point called an interaction point.
A planetary or extra planetary system in our galaxy or even a far away galaxy is studied through huge telescopes that are mounted on high altitudes at different location of our planet or telescopes that are made to orbit around the earth. Telescopes can be sent to any interplanetary configuration and they send us useful data of the experiment they are set to study.
One very common aspect of all these experiments and systems is they can be studied without having to do realistic experiment. This is done through a conception (and conceptualization) known as simulation. Simulation in crude connotation therefore means mimicking a real system. If I have a molecular system that I want to study I can study its behavior by simulating the whole molecular system. Its useful properties, which are known and its modeled behavior together, are called a simulation.
Therefore simulation is a useful and often followed practice in any modern scientific study. In most simulation studies the experimental data known previously to a new experiment, data from previous time stamps, from previous experiments, from previous samples that are subsets of this new experiment they all are added into one body of reference and this reference is called a simulation of the system which is under study.
So in particle physics experiments all knowledge gained about a particular particle or its interactions in a detector previous to a new experiment or measurement are collected as simulation information. This is usually called a Monte Carlo simulation to represent the fact that particle behaviors are reproduced through generating random variables to simulate the particle configurations such as its spin and its direction. In addition to simulating the nature of these particle processes one also adds into the simulation all the gory details about what happens when the particle is actually made to pass through the detector material.
The detector material is usually simulated through its geometry and material properties, eg at what angle a given particle will pass and what will be its impact on the particular matter of that part of the detector. The matter could be a glass, iron, tungsten, silicon or scintillators. All the properties of these materials when a particular type of particle passes through these and there are 100s of different kinds of the particle, are noted and passed on to the database or the simulation algorithm.
So a simulation (Monte Carlo in short in particle physics terminology) package in particle physics is a quite complicated note with literally 100s thousands of variables and thousands of ntuples, columns and tables. One can use these simulation tables to study in advance the expected behavior of a system of particles and their interaction in the space inside a particle (accelerator) detector the size of which may be as large as a small library room.