An independent contractor is a person who an employer hires to complete a project. The employer does not have control over how the project is completed. In California, a person hired to do something for the benefit of another is assumed to be an employee unless the employer proves otherwise. See, Labor Code, § 3357.
For example, if a small donut shop hires a web designer to build a website for the business and agrees to pay a set amount for the project, then the web designer is an independent contractor and not an employee.
As discussed here and here, whether someone is an employee or an independent contractor involves a number of factors.