There is a lot of confusion when you begin the bonsai journey. You'll hear suggestions from everyone about how to do everything. One thing that helps is understanding why you're doing something. Understanding the physiology of your plants will help dramatically with that.
I wanted to start with this photo that has a bit of information. You don't have to memorize this to practice bonsai by any means, but I'll go over the important parts.
Koning, Ross E. 1994. Plant Basics. Plant Physiology Information Website. http://plantphys.info/plant_physiology/plantbasics1.shtml. (6-2-2018)
Trees need 4 things to survive:
Leaves are responsible for the sunlight and movement of water. Leaves photosynthesize and send the sugars and starches created to the plant via the phloem. As water evaporates in the leaves the process of transpiration occurs. Transpiration in essence "pulls" a chain of water molecules from the roots up to the leaf to replace the loss of water. The cuticle is a waxy covering on the leaf that stops moisture loss once the leaf has fully "developed." Once a leaf has its cuticle it begins photosynthesizing and accumulating energy. This happens once a leaf has reached its mature size after breaking out of the bud, changed to a darker color, and has become less flexible.
There are several techniques used in bonsai that involve leaves, namely: full and partial defoliation, leaf cutting, pruning, watering, development of branches and trunk, and allocation of resources throughout the tree.
Branches are like roads, the more they are used the bigger they get. If one branch has 20 leaves and another has 5, the branch with 20 leaves will thicken quicker than the branch with 5 leaves. This is because more energy is being created and more water is being moved there from the water loss of the leaves. We learn from this that foliar mass = girth.
Roots are responsible for the uptake of water, oxygen and nutrients. There needs to be a balance of water and oxygen in the soil for this to take place and not starve the plant for either water or oxygen. Roots move water and nutrients to the plant through the xylem.
Large roots help to anchor trees in nature. Small fine roots are the main roots that absorb moisture and nutrients. Roots function the same way branches do in the sense that the more fine roots you have attached to a larger root, the bigger it will get.
In bonsai we repot anywhere from every year to every 10 years depending on the needs of the tree. We prune the roots and style them much like we do to the branches above the soil line.
The trunk and branches have bark which aids in retaining moisture. I'll talk more about the stem as needed in further discussions, but what you need to know is: the xylem moves water and nutrients from roots to leaves, phloem moves energy to the tree from the leaves, and cambium helps the tree get thicker and indicates the tree is still living by showing a green tissue layer under the bark.
Wiring and bending a tree can be more difficult depending on the species of the tree. Different species have varying abilities to bend, and this is due to the changes in the xylem, phloem, cambium, amount of water moved (water loss due to transpiration), and bark.