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:
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:
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:
Looks like PETG doesn't break but bends at much higher weight loads then PLA which breaks relativity easy.