what is resource units in Microsoft Office Project 2007

Sometimes it takes one resource to complete a task, but other times you need more. Sometimes a resource can only work part time. To account for these differences, Microsoft Office Project 2007 uses units (units: The quantity of a resource assigned to a task. The maximum units is the maximum number of units available for the resource. For example, if you have three plumbers, the maximum units is 300 percent or three plumbers working full-time.) to calculate the exact amount of time resources can work on a task............
Details: http://office.microsoft.com/en-us/project/HA102548291033.aspx