The difference is best explained on Plans & Pricing page. In short, the free ScreenshotMonitor version is limited 2 employees, 3 screenshots per hour, the screenshot images are kept for 2 weeks. Standard version is 6 screenshots per hour, the screenshot images are kept for 6 months. And Professional is up to 30 screenshots per hour, screenshots are kept for 1 year.
There is also a metered pricing when you pay only for hours logged independent of the number of employees.