Pallets come in a HUGE range of sizes, designs and wood types. This is because they are created and assembled to carry different types of materials with different weights. Most pallets are made of pine (soft wood) but some are made of hard wood. The hardwood ones are much harder to dismantle but are worth the effort if you are making outdoor projects, as hardwood fairs much better than softwood in weather.
Pallets are comprised of slats and stringers. The stringers are the support pieces. There are usually two or three of these per pallet. Stringers can be thick planks of wood about 10cm x 4cm, or they can be made of square blocks with slats attached to them. The slats are the long top pieces that are connected to the stringers. Slats can vary in width and thickness.
The Australian pallets usually have thick (4cm) wood stringers, whereas the European pallets usually have stringers made out of the slats, with compressed chipboard blocks or wood chunks in between them. The image below, sourced from 1001pallets.com, shows some common types of pallets:
Pallets are either chemically treated or heat treated for bugs and pests. Be sure to use the heat treated ones, which will have “HT” printed on the side.