Wiring is a very important technique used to train and style Bonsai trees.
Wrapping wire around the branches of a tree allows you to bend and reposition the branches to your liking.
Two kinds of wire are used for Bonsai, anodized aluminum and annealed copper. Aluminum wire is usually used for deciduous species. Copper wire is used for conifers.
If you're a beginner I advise you to use the anodized aluminum wire as it's is easier to work with.