There are many different variants of polyethylene, and material properties vary across them. Also, low and high density polyethylene (LDPE and HDPE respectively) are two of most common amongst them.
LDPE: LDPE is the plastic good to make plastic bags for grocery stores. It has a high ductility but a low tensile strength.
HDPE: A stiff plastic used for more robust plastic packaging, like laundry detergent containers, the containers used for construction applications, and trash bins.
UHMW: Ultra-high-molecular-weight-polyethylene is an extremely strong plastic that can rival or even exceed steel in strength, and good for purposes that include medical applications (e.g. artificial hips).