Flexible polymer frames with foam padding, mesh screen visor and an elasticized adjustable strap.
Transparent polymer screen materials can have an annoying tendency to fog up with the humidity of humid environments or even just the body heat and exhaled breath of the user. Mesh materials have an inherent advantage of being completely immune to fogging.
The downside to mesh, primarily, is that mesh does not offer 100% protection. With lower powered BB strikes it will do fine but anything mid or high power can result in struck BBs breaking and shattering. Breaking BBs cast of small fragments and while these may have less energy then the initial BB they can still do damage to un protected eyes. The mesh is full of holes increases the chance of high speed BB strikes shatter fragments passing through the gaps.
Mesh also fails to stop environmental secondary annoyances as well. Transparent polymer lenses keep the wind out of your eyes as well as offering coverage from rain, smoke, hail and a whole host of other miscellaneous factors that can be found on the play field. Mesh protection however is not air or liquid tight; they can cut down on air flow but will not stop wind and offer no protection from rain or smoke. The counterbalance is that the airflow through mesh makes the breathability factor more comfortable over longer periods of time then the almost air tight transparent polymer goggles.
Transparent lenses also offer god visibility whereas mesh visors block a lot of light which reduces your perception. In low light and night games, mesh visors are just a step away from blindfolds.
For their shortcomings, mesh goggles are also incredibly dirt cheap by common standards. If you are just playing very low powered spring pistol games in the backyard then these goggle are fantastically cheap, comfortable and robust enough for casual use.
Pay close attention to your protection; eye protection is serious business in Airsoft, no body wants to end up blinded. We are all for budget conscious players but do not skimp on the eye protection; be absolutely sure to select the correct choice for your gaming.