The bags marked as "Gusseted Bags" are shaped with an expanding piece of material on the sides (the gusset) that allows for the bags to be expanded more and hold a higher volume. The Flat Bags are more like two pieces of plastic fused together and cannot hold the same volume. Your choice between "flat" vs "gusset" largely depends on the shape and size of products you're placing inside.
Poly Bags vs BOPP Bags
The difference between these types is in the plastic used to create them. BOPP stands for Bi-Oriented Polypropylene. It has excellent oxygen and moisture barrier properties and use an ASBX (Acrylic Saran) coating to provide longer shelf life. Both of these types are heat sealable and oil, grease and puncture resistant. Both types of plastic are also FDA approved for safe food contact. The thickness ranges between 1.1 and 1.5 mil.
Clear Slider Top Bags
These clear plastic food bags provide an easy to use "slider top" to engage the interlocking top strips that provide an airtight seal. The slider top is a more user friendly closure versus the heat seal method needed for bags without a closure. These slider top bags are constructed using 3 mil (thickness) low density polyethylene (LDPE). They're FDA / USDA approved for direct food contact, as well as rated for 120° F temperatures.