Bonsai is just a tree in a pot, to be more specific, a bonsai is a miniature representation of a full grown tree cultivated in an aesthetically compatible container.
Any species of tree has the potential to be a bonsai although some are better than others. A misconception is that dwarf varieties make the best ones but this is not the case as they are often weak growers and easy get disease. The foliage needs to match the scale of the tree.developing a bonsai takes a long time and discipline: daily watering regular, feeding, trimming, repotting, wire-training and root care.
Beech, hawthorn, pine, larch and elm make fine bonsai so do many garden shrubs. Bonsai are among the best cared- for plants in cultivation and often out live their full size counterparts.