Service Providers meeting SLA's

Hello everyone,

As I'm fresh in networking industry then my questions are probably very basic, sorry if that's the case. My questions are: What goes into SLA in terms like bandwidth, uptime, delay? How do providers and customers track if expectations are met? How is it different for providers doing it for companies to those doing it to end customers or anything else that follows customer/provider relationship ?

Looking forward to Your responses :)