Object Styles are a combination of a Plugin Object and a Symbol. When you place several Plugin objects in your drawing, they are all individuals. When you make changes to one of them, it does not affect any of the others. A symbol in the drawing is an exact copy of the one in the Resource Manager. When you place several in the drawing they are all the same. When you edit the symbol definition, they all update.
An Object Style is part way between a plugin object and a symbol. The object style can control all of the parts of the plugin, or just a few, you choose. You can make it more like a plugin and have more flexibility, or you can make it more like a symbol and have less flexibility.