There are 11 new "PatchOperation" actions we can perform to modify Defs without copying the whole thing. This in turn improves mod compatability dramatically - it allows two mods to modify different parts of the same Def without conflict. This guide only describes the basics of each operation available to us.
Patches go in xml files in a subfolder named "Patches" in your mod folder (So "MyMod\Patches", just like you might have "MyMod\Defs" or "MyMod\Textures"). Example.
Each PatchOperation has an "xpath" node, which should contain an xpath selector for the xml node(s) that the operation should affect. This is not an xpath tutorial. If you don't know xpath, not my problem. But minimurgle has you covered!
To illustrate the operations, I'll use this simple example def:
<DesignationCategoryDef>