Per IRS Publication 15-B Employer's Tax Guide to Fringe Benefits, page 3:
A fringe benefit is a form of pay for the performance of services. For example, you provide an employee with a fringe benefit when you allow the employee to use a business vehicle to commute to and from work.
Any fringe benefit you provide is taxable and must be included in the recipient's pay unless the law specifically excludes it. Section 2, page 5, discusses the exclusions that apply to certain fringe benefits. Any benefit not excluded under the rules discussed in section 2 is taxable.
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