The most common thermoplastic used in personal 3D printing has always been the oil-based ABS though recently its popularity seems to have been matched by the bio-degradable PLA.
Though ABS and PLA are widespread, easy to find, inexpensive and suitable for most projects, they are not the only options. Many other filaments are available for the 3d printing enthusiast and each one has unique mechanical properties worth knowing.
As you can imagine, "flex" is short for "flexible". We are basically talking about a material that can be easily bent and squeezed without breaking.
PET is strong and it is recommended whenever you need a strong and durable part.
I encourage you to try all available types of filament so you can fully experience the possibilities that the 3D printing technology currently offers. In all cases, keep in mind that each filament type (and even each brand) may have specific requirements in terms of printing temperature, printing speed and even spool storage.