You have good grass seed or sod in place, you've fertilized and watered, and you can crosscut like a golf course summer greens crew. So what's the one extra thing you can do to bump up the overall aesthetic of your yard? Edging.
Edging contains and separates the different elements of your yard from each other and will give your property a defined and considered look. Start out by having a look at your yard. Consider it like a canvas. There are the walkways, flowerbeds, tree plots, maybe a patio, and the sidewalk. If you're starting your landscaping from scratch, consider using landscape fabric as an underlay to help with week control in your gardens and in the areas you edge.
A flat spade does a better job than a shovel. You can use string as a guide to ensure your lines are straight, but eyeballing it works too. Simply cut a V-shaped trench out of the lawn, leaving you with a gap between the grass and the area you're edging. You want the gap to be around 3 to 4 inches, depending on what you have in mind. To keep the edge sharp it should be re-cut once every month or so.
Select the material you want to edge each area with. One idea is to go for natural materials like wood or mulch next to hard-scape surfaces like asphalt and concrete, while the borders around your organic beds, like gardens and tree wells, can benefit from 'man made' materials like brick or decorative rock. However, there's no reason why you couldn't use whatever fill material you wanted to in each place.