The fast simulation of a high energy physics experiment is a tool used by experimentalists to quickly assess the potentiality of their detectors on a specific analysis or reconstruction tecnique, before imbarking themselves into a more time- and CPU-expensive detailed study with the full simulation. In some cases, it can also be considered the access point for theoreticians wanting to see "how do their model looks like in the real life''. The aim of this contribution is to introduce how fast simulations work in the ATLAS and CMS experiments at LHC, and which are the main differences with respect to a full simulation. A comprehensive comparison of a few results obtained with the full and the fast simulation in CMS is also given, in order to provide an example of application of the two methods
