For normal SQL statements, either a / on a line by itself, or a ; at the end of the command, will work fine.
For statements that include PL/SQL code, such as CREATE FUNCTION, CREATE PROCEDURE, CREATE PACKAGE, CREATE TYPE, or anonymous blocks (DECLARE/BEGIN/END), a ; will not execute the command. Since PL/SQL uses semicolons as line terminators, its use as a command terminator must be suppressed in these statements. So in these cases, you must use / to execute the command.
In my experience, people prefer to use the semicolon when possible and use the slash only when required.
Note that for SQLPlus client commands — such as SET or EXECUTE — no command terminator is necessary at all, although people often end them with a semicolon out of habit.