A service-level agreement (SLA) is a document that spells out two or more parties' rights and obligations under a contract for work (such as between a company and its service provider). The main purpose of an SLA is to spell out the level of service that will be provided under the agreement. An internal IT services organization may also provide an SLA to its internal business "customers." An SLA can comprise a few short pages up to a few hundred pages. The basic components are a statement of the parties' intent, an outline of the responsibilities of each party (including acceptable performance parameters with applicable metrics), a statement on the expected duration of the agreement, a description of the applications and services covered by the agreement, procedures for monitoring the service levels, a schedule for remediation of outages and associated penalties, and problem-resolution procedures.
Source: CIO, http://cio.com