Magnetic reconnection has been recognized to be the mechanism allowing energy stored in magnetic fields to be released abruptly, solar flares, coronal mass ejections (CMEs) and substorms being archetypal natural plasma examples. A full understanding of magnetic reconnection requires to explain how the energy is stored in the magnetic field to be then suddenly released and converted into particle acceleration and heating. I will first discuss the mechanism for the onset of fast magnetic reconnection and recent development on the fractal reconnection and plasmoid instability, with possible application to substorm onset. In a second part of the talk by means of 2D, full-particle simulations in an open system, I will show the result of the studies of energy conversion for different guide field intensity. In particular I will discuss the threshold guide field separating when parallel and perpendicular energy transfers dominate, confirming recent MRX results, in agreement with MMS observations. We studied the electron distribution functions and self consistently evolved particles orbits for high guide field configuration, investigating possible mechanisms for electron perpendicular heating. Finally I will discuss jet features in guide field configurations.