Everyone experienced with this art has a different idea of what is aesthetically pleasing in a Bonsai tree, you don't need to take everything they say as "set in stone" or desperately important, and should keep in mind that these are just the preferences of these people. I'm sorry to say that, if you were to follow everybody's advice while training a Bonsai, your poor plant would wind up very confused.

The two basic styles of Bonsai are koten or classic, and bunjin or comic. The two are different because the koten fixtures a broad base and narrows off at the top, while the other does just precisely the opposite, narrowing at the bottom and widening at the top.

However, though these are the two most basic styles of Bonsai, there are also many subdivisions you can train your plants into. If you have experience with the species of tree, you can make your own decisions on which styles to train it into. If not, visit a website or read a book to get a better idea.

Once you have chosen a style, don't feel restricted by the rules of the style. You can bend or mold them to best fit your tree or adapt them to better please your personal aesthetic sense. Also, consider how your tree looks in the wild, and try using some of those aspects as you train your tree.

There are five main styles; Formal Upright, Informal Upright, Slanting, Semi-Cascade, and Cascade.

Formal Upright

Formal Upright requires near perfect conditions, just the right amount of sun, water, and air. It can be done with a variety of species such as Larches, Juniors, Pines, and Spruces. Maples are usable but not the best choice. Formal Upright Bonsai have perfectly straight trunks with evenly spaced branches.

Informal Upright

Almost all plant species can be used for Informal Upright. The trunk should slant to the left or right but never towards the viewer. The trunk can make unpredictable twists and bends, just as long as the tip of the tree lines up over the base of the tree (where the trunk comes out of the soil).

This is a picture of a slanting-style Bonsai tree, courtesy of Webshots.com

Slanting Style

Whether curved or straight the trunk of a slanted-style Bonsai goes off at a definite age, differentiating it from Informal Upright. Many of the same trees used in Informal Upright are also good for use here, though Conifers work quite well. This style is my personal favorite.

Semi-Cascade

The "growing tip" of this tree grows sideways rather than down or up or slanted. It doesn't go past the base of the pot. Flowering cedars, cherries and junipers can work very well for this, though many can be used.

Cascade

The growing tip of this tree reaches below the base of its container. The trunk has a natural taper, and the branches seem as if stretching toward a distant light source. The trunk resembles a stream tumbling over mossy rocks. Pick trees that naturally have a flexible trunk.