stackoverflow-question about this
The hard real-time definition considers any missed deadline to be a system failure. This scheduling is used extensively in mission critical systems where failure to conform to timing constraints results in a loss of life or property.
The firm real-time definition allows for infrequently missed deadlines. In these applications the system can survive task failures so long as they are adequately spaced, however the value of the task's completion drops to zero or becomes impossible.
The soft real-time definition allows for frequently missed deadlines, and as long as tasks are timely executed their results continue to have value. Completed tasks may have increasing value up to the deadline and decreasing value past it.