It is universally understood that old tree trunks and branches mainly consist of wood and not much of the softer stuff. To copy or create age in Bonsai, it is therefore desirable to have as much wood and bark as possible. These things do come with age and is very desirable in Bonsai, especially the bark, as that is the most visible part on trunks and branches and even on exposed roots.
So what is wood? If you start peeling off the outer layers of a tree trunk or branch, you will find two kinds of wood underneath these top layers. Closest to the bark or outer layer is the sapwood. This is moist and a living layer consisting of the xylem fibers. These are tubes which help the tree to transport water and nutrients from the roots to the leaves. The mechanism for this is explained in other posts on this site.
Underneath this layer, you will find the harder layers called the heartwood. This is dead material and basically consists of the xylem tubes which are not hardened by resins and no longer transport water and nutrients. This is the wood of the tree and is easily recognizable by the annual rings.
Back to the external layers. On the outside of the sapwood you will find the cambium and this is where the growth takes place, thickening the tree and the source of the annual rings. On the outside of this, you will find bark forming. This is basically just the original epidermis (outer layer of cells of a young plant) that undergo chemical and structural changes over time to form the woody or corky dead tissue that we know as bark. This is brittle and not well fastened to underlying layers.
One of the important things to remember for Bonsai is that when we bend a trunk quite severely, you might here a bit of cracking going on. This is usually the deadwood and as long as you do not snap it completely, it does not interfere with the living parts of the tree, merely the structural bits. What you need to be careful of is to not break the outer layers too much as that is where the water and nutrients are being transported through.
To form a permanent bend in a branch or trunk, the wire will assist to keep the branch in position for the resins and dead fibres in a tree to settle in the new position and then hold it there. This is why you need to keep the wire on for a s long as possible, but at the same time be careful of the outer layers still growing and growing around the wire. Technically it is not the wire biting in, more a case of the outer layers growing around the wire.
We also have to be very careful with the bark. As it is an outer layer and not very well attached to the underlying layers of a very old tree, it can come off very easily. As pointed out earlier, it is desirable to have the bark on trees like oaks, redwoods and others, it needs to be protected. A few things to do is to never pick a tree up by its trunk, cover the bark with rafia when applying wire and protect the bark with sifter material when using guy wires.
Protect bark from guy wires. 
Raffia.
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