The difference between "-" and "no hyphen" is that the latter keeps your existing environment (variables, etc); the former creates a new environment (with the settings of the actual user, not your own).
带横杠的会创建一个全新的环境，使用的是真实用户的设置。 不带横杠的还保留了之前用户的设置，例如当前目录，环境变量等等。