What does teflon give on decomposition?
Like any other polymer decomposition process, the products of PTFE decomposition depends on the chemical species present while PTFE is undergoing the process and temperatures.

General process is this : Decomposition  is  initiated  by  random-chain
scission, followed by depolymerization. Termination is by
dis-proportionation. And all of this happens rapidly above 600 K (~326 C). There are no other conditions applied for the decomposition to take place hence can happen in dry or aqueous environments, which would give different by products.  Cooking below 200 C, it would be completely safe since the mass loss below 300 C is undetectable. Only above the glass transition temperature (~326 C) is the mass loss significant. 

Decomposition products will depend on the environment and usually 
Oxygen usually does not enter the cycle directly but through water, to give species like, 


Carbonyl fluoride 
Carbonyl difluoride


As expected other species like fluorinated alkanes and alkenes are obtained like,


the monomer, tetrafluoroethylene
Hexafluoroethane 
Octafluorocyclobutane
Octafluoroisobutylene 
Perfluoroisobutylene 
Tetrafluoroethylene 
and more.


However, the chemicals (species) listed above are for controlled lab experiments. What happens in real life cooking scenario is not (or cannot) be anticipated. But, it can be readily said that the above species can react with other chemicals in food to give fluorinated compounds that will be harmful for humans. As a precaution, one should not cook above 200 C on PTFE coated utensils to be completely safe.

Check this extremely detailed page for abstracts and toxicity remarks from research papers on PTFE decomposition on Fluoridealert.org where the conditions and level of toxicity is reported in an organized manner. 

Polymer decompositions- check PTFE section for general comments.