The IPC (Institute for Printed Circuits) is an association focusing on standardizing PCB design specifications, material specifications, inspection specifications as well as regulation of emerging technologies such as rigid flex PCBs etc. They are most commonly known for their guidelines regarding the size of pads for surface mount footprints.
However, in my personal experience, I usually give priority to the footprint advice (recommended pad layout, size etc) provided by the manufacturer when I'm making "exotic" footprints myself. For standard footprints (QFN, SOIC, TSSOP, ...) I use medium density footprints from standard footprint libraries.
Ultimately, it is the PCB manufacturer that decides how much clearance you must have between pads or tracks, how thick tracks must be etc.