A Service Level Agreement, or SLA, is an agreement between a service provider and a customer that determines the initial response and completion due time and date for the resolution of issues.
SAP Service Cloud has this functionality for the key dates in tickets it receives to be determined by an SLA - what the initial response and completion due time and date for a given ticket's resolution should be. You can have varying SLAs with varying key dates.
What happens then, is that SLAs can be created in the system that the system can then assign to tickets as they are received. For example, Platinum, Gold and Silver. A ticket with a given SLA when it is received will then automatically start counting down against the set key dates. As a result, tickets receive an initial response by a certain time or date, and should be solved by another time and date.
You can define rules to determine which SLA should be applied to each ticket. That means that for example, a ticket coming from Germany can be treated with an SLA based on the German holiday calendar, while tickets coming from the US, will be treated with an SLA based on the US holiday calendar. Finally, the ticket can be routed to the appropriate team based on work distribution rules.