PETG is a 3d printing filament made from PET-G which is Glycol-modified PET, the modification is done by adding a second glycol during polymerization.
The resulting molecular structure is irregular and the resin is clear and amorphous with a glass transition temperature of 88 C (190 F).
Filament made from it gives good combination of toughness, clarity and stiffness
More about PET-G:
http://en.wikipedia.org/wiki/Polyethylene_terephthalate
Here are videos from Mihai Oltean showing actual 3d printing with PETG and object properties:
Here are links to his blogposts about his PETG experiences with more details:
http://3d-printizer.com/blog/?p=258
From his experience PETG doesn't stick to blue tape or to cool glass printbed.
Detailed post about testing strength of PLA vs. PETG printed objects:
http://3d-printizer.com/blog/?p=268
Looks like PETG doesn't break but bends at much higher weight loads then PLA which breaks relativity easy.