Uptime is the amount of time that a service is available and operational, typically expressed in percentage.
A service-level agreement (SLA) defines the level of service you expect from a vendor, laying out the metrics by which service is measured, as well as remedies or penalties should agreed-on service levels not be achieved.
Redundancy, monitoring and alerting are key to ensure a safe and reliable service.
99.9% uptime equals to a period of downtime of 1 minute and 26 seconds per day.
99.99% uptime equals to a period of downtime of less than 9 seconds per day.
99.999% uptime equals to a period of downtime of less than 1 second per day.
See also: Three nines, Four nines, Nine nines, SLA calculator, Downtime calculator, Uptime calculator.