SAP Crystal Server is an entry-level BI solution based on the SAP BusinessObjects Business Intelligence Platform.
Crystal Server's Getting Started Guide describes Crystal Server as:
[..] an entry-level BI solution that enables you to securely open, view, interact with and share reports and dashboards over the Web. It enables you to intuitively explore and visualize data. Built on the SAP BusinessObjects Business Intelligence Platform, Crystal Server includes a report publisher, a web portal interface that displays reports, and Software Development Kit (SDK) that allows you to customize Crystal Server and integrate it with your custom applications.
Do not confuse Crystal Server with Crystal Reports!
Documentation can be found on SAP's help site. Currently, there's documentation for Crystal Server 2011 and Crystal Server 2013.