Outdoor Grill

Cast-aluminum
Lightweight and rust-proof, gas grills made of cast-aluminum are often entry-level or budget grills. With that said, cast-aluminum is very durable and retains heat very well. Furthermore, the rust-proof aluminum typically requires less maintenance than grills made with other materials.
Common components: Grill body (burner box and/or grill lid)

Powder-coated steel
Powder-coated steel is covered with a protective coating of paint and is most commonly used on grill hoods and bodies. Available in a variety of colors including black, red, blue, and many others, it is a popular grill material that adds a flash of style among a sea of black and stainless steel options. As long as the protective coating remains intact, powdered-coated steel is very durable and relatively easy to clean, making it a more affordable and visually appealing alternative to the ubiquitous stainless steel.
Common components: Grill hood and body

Stainless steel
Stainless steel is one of the most commonly used BBQ grill materials, but you will find different grades or quality levels at different price points. The different grades indicate the mixture of metals used to make that specific steel. They all must have at least 11% chromium to be considered "stainless", but grills that utilize lower quality stainless steel will be cheaper and typically won't mention a specific grade number.
Common components: Grill body, hood, grill grates, hinges, burners, bezels, carts

Cast iron
Cast iron can be good and bad in a gas grill, depending on the grill owner. Because it is heavy and porous and therefore requires regular seasoning or a ceramic/enamel coating for maximum longevity, cast iron is typically only used for gas grill cooking grates. This adds to the overall weight of the grill, so if portability is a concern, look for lighter materials like stainless steel. Cast iron takes longer to heat up than other metals, but holds its heat very well once it's up to temperature, and is the best material for achieving grill marks and a perfect sear.
Common components: Grill grates

Plastic
Many entry- and mid-level grills have exterior plastic components, such as knobs, bezels, and hooks. While plastic components are fairly durable, over time exposure to the elements will break down the plastic, making it more brittle and prone to cracking. To avoid pre-emptive damage, make sure you are covering your grill when it isn't in use and storing it properly in the winter.
Common components: Knobs, bezels, and hooks.