Formal gardens and most normal gardens, today , have distinct boarders and edging between features.
Paths almost always have a distinct edge. Transitions from lawn to planter often have a special edge treatment. From a flower bed to a lower care planting of perennial shrubs and trees will often have some sort of border to divide the two areas.
Borders and edge treatments serve a number of purposes but chief among them is to keep things where they belong. A lawn can become an invasive pest if it starts to grow into the annual flower garden. Similarly, you don’t want your gravel path migrating into your lawn where your mower will have the opportunity to turn into a machine gun, shooting the small stones all over the yard.
There are more choices than you can imagine when looking for edging or boarder treatments. First, you have to consider whether you want a sharp or a soft edge. If you are drawing the line between a garden path and a lawn, typically, you will want a very distinct edge. If, on the other hand, you have a quiet bark or natural stone path through your shrubs and trees, you might want a much softer transition.
Here are some of your options listed from the distinct, sharp edge to the softer, less definitive edge effect:
Concrete curbing - This is a great choice around a lawn. It is easier to cut and trim against a durable concrete edge. It can hold up under the pressure of a lawnmower every week. One of the nice new things about concrete edging is color. No longer are you stuck with a concrete grey curb. You can choose from colors as dark as lamp black to warmer tones of brown and red. You can even stamp concrete curbs to look like stone or brick. 
Brick or paver edging – In the past, setting the bricks or pavers was a real challenge. Today products like Diamond-lok and others allow you to easily set the pavers or bricks and keep them in place under foot and machine traffic. Many paths are now constructed with the sharp edge of pavers in a soldier’s course on the sides of a more natural look like gravel or bark between the borders. 
Natural Stone edging – Some paths are created with vertically placed pieces of flagstone as edging while others may use river rock more as a visual than a physical barrier on the edges of a path. The beauty of using the river rock is you can place it casually and let it move as the path is used. Its lack of a formal look means that if a stone is moved slightly out of place it doesn’t look out of place.
Wood edging – Bender board is the old standby. The plusses include ease of placement and the ability to form nice smooth curves. One downside is bender board’s short lifetime. It breaks easily when stepped on or run over with a mower, and, it is hard to get it made of quality cedar or redwood that will last more than a couple of years. Another downside is the cost. When viewed over its short lifetime, bender board can be more costly than most of the stone and concrete alternatives.
Steel edging – Though rising in cost with the price of steel, this is a product with a long life, can contain almost any material or planting, is easy to install, and can be made almost invisible while holding a sharp edge. This is a personal favorite of mine when combined with a simple quarter minus gravel path.
Plastic edging – This is a reasonable alternative to steel edging with one advantage and one major disadvantage. Advantage? It is less expensive. Disadvantage? It doesn’t hold its shape as well and often, after only a few months in place will move with the soil and look very uneven and unprofessional.
As with any building project, the preparation is essential to the success of the project. With the possible exception of a casual garden path of bark defined by edging of natural stone, any edging treatment in your landscape will require proper excavation, tamping, and careful construction.