There are many options when it comes to bordering your grass. What became extremely popular in the 90's was concrete curbing. Before that, and now, many types of materials were used to border grass including: rocks, plastic edging, bricks, wood, and rubber.
And in some cases there is no physical barrier which requires diligent work to keep a clean edge, like the photo below-
In our opinion, the closer we can be to a natural look, the better. We prefer to not have a visible barrier, like the ones mentioned above. We believe that simple, clean, and natural-looking edges are the absolute best way to make and keep grass borders healthy and attractive.