UPDATE: The White House announced July 2 that the employer mandate has been delayed until 2015.
Generally speaking, employers with more than 50 full-time employees must provide insurance coverage for their employees, beginning in 2015. The cost of that insurance can't be more than 9.5% of an employee's annual pay.
Even if you have more than 50 full-time employees, you might choose not to provide insurance. Beginning in 2015, you'll pay a penalty that will help fund the subsidies many individuals will use to help pay for their health insurance.