什么是SaaS(翻译自维基百科)

软件即服务(SaaS),有时被称为“按需提供软件”，指的是一种软件供应模式，这种模式下软件和相关的数据都被集中的托管到云端。用户常常用瘦客户端通过web浏览器访问SaaS服务。
SaaS已经成为大多数商务应用的常见供应模式，包括统计，协作，客户关系管理(CRM)，管理信息系统(MIS)，企业资源计划(ERP)，发票，人力资源管理(HRM)，内容管理(CM)和服务台管理。SaaS模式已经被用于所有顶级企业软件公司的战略部署。
很多人认为SaaS一词是云计算术语的一部分，其他部分包括IaaS(设施即服务)，PaaS(平台即服务)。

商务应用的集中式托管服务追溯至19世纪60年代，那时被服务供应商们称作共享时间或效用计算。这种服务通过全世界的数据中心，为大型组织，如银行，提供计算能力和存储能力。

到了19世纪90年代，互联网的扩张带来一种新型集中式计算服务，称为应用服务提供者(ASP)，ASP通过托管专业的商务应用来提供业务，其目的是通过这种对商务应用的集中式管理和专业性维护来降低商务的成本。

Saas延生了ASP模型的思路，其应用场合更加明确:

used on managing and hosting third-party independent
software vendors' software, as of 2012software-as-a-service vendors typically develop and manage their own software
whereas many initial application service providers offered more traditional client-server applications,
which require installation of software on users' personal computers, contemporary software-as-a-service
solutions rely predominantly on the Web and only require an internet browser to use
whereas the software architecture used by most initial
application service providers mandated maintaining a separate instance of the application for each business, as of 2012 software-as-a-service solutions normally utilize a multi-tenant architecture,
in which the application serves multiple businesses and users, and partitions its data accordingly

The SAAS acronym allegedly first appeared in an article called "Strategic Backgrounder: Software As A Service", internally published in February 2001 by the Software & Information Industry's (SIIA) eBusiness Division.[7]

DbaaS (Database as a Service) has emerged as a sub-variety of SaaS.[8]