Quick answer
Edging creates a sharp vertical line where grass meets sidewalks, driveways, and beds. Trimming (string trimming) cuts grass around obstacles a mower can't reach. Both give a finished look.
Edging
Use a stick edger or blade to cut a vertical line. In Bermuda lawns, regular edging keeps runners from creeping over concrete.
Trimming
String trimmers handle fence lines, trees, mailboxes, and tight corners. Keep the line away from tree trunks to avoid bark damage.
How Often
During peak Bermuda growth, edging every mow or every other mow keeps lines crisp.