SQL Server Reporting Services (SSRS) is a competitor to Crystal Reports. It's a way of defining reports, just like Crystal Reports, and an engine for running those reports.
SSRS itself is a website that hosts these reports and can serve them to users through a web interface, or can schedule them for delivery over e-mail.
As a programmer, I prefer SSRS over Crystal due, among many other things, to the fact that it's more stable and uses an XML-based report file format, unlike Crystal's old binary format. That means that certain design tasks can be done simply by editing XML if it comes down to it.
Read all about it in the SQL Server Books Online.
It's more or less the same thing as Crystal Reports - a reporting engine and designer. The SSRS has some benefits over Crystal: it's server-based, e.g. you can have your reports be handled and prepared on the server (e.g. on a nightly schedule, and distribute them by e-mail or something) - but also allows local execution of reports.
Due to the fact it's a Microsoft product, it's more tightly integrated with SQL Server and Windows than Crystal Reports.
And if you have a valid SQL Server license, you have Reporting Services - it's a free add-on to SQL Server.
It's the reporting solution offered by Microsoft. You get it included if you have a MS SQL server license. Wiki gives more detail: http://en.wikipedia.org/wiki/SQL_Server_Reporting_Services
And here's more info from Microsoft: http://www.microsoft.com/sqlserver/2008/en/us/reporting.aspx
Basically, you install the Report Server software, which runs a website and webservices that you can then use to host reports. The reports themselves are built using Visual Studio, and quite easy to create. Saying that, you CAN make very complex reports by using multiple queries and scripting.
As Dave mentioned, it's got pretty good integration with the whole Microsoft Stack.
HTH, Lance
