Particle System
The Particle System component simulates fluid media like liquids, clouds and anything related to fire by generating and animating a number of small 2D images in the scene. For a more detailed introduction to particle systems and how they can be used, see the documentation page.
Properties
The Particle System component has many properties, and for convenience, the inspector arranges them into collapsible sections or "modules" described below. Each section can be expanded and collapsed by clicking on its header. On the left-hand side are checkboxes which can be ticked to enable or disable certain settings. For example, if you don't want to change particle size during emission, you can simply uncheck Size over lifetime.
In addition to the modules, there are several other controls in the Inspector. The Open Editor button displays the options in a separate editor window, which also allows you to edit several systems at once. And the Resimulate checkbox is responsible for whether the property changes will or won't be immediately applied to the particles just created by the system (alternatively, the existing particles will disappear, since they will be the only new particles whose properties have been changed). The Wireframe button displays outlines of mash objects to show the particles behind them.