An SLA is a Service Level Agreement. It's a contract between a company and its customers that defines the level of service the company will provide. The agreement can be between two companies, or between a company and an individual customer. The agreement should spell out what services the company will provide, how often they will be provided, and what standards the company will meet. It should also include a mechanism for measuring whether or not those standards are being met, and for resolving any problems that arise.