These bags are most commonly made of HDPE (Plastic #2) but may also be made of LDPE (Plastic #4) or non-recyclable plastic like PLA (Polylactic acid - a kind of plastic #7).  The see-through bags tend to be LDPE and the opaque ones HDPE.

Although both HDPE and LDPE are technically recyclable, it is hard to sort the material for recycling so tends to have very low value as a waste plastic.