Are you asking as an interviewer, or the interviewee?
If you are interviewing for a job as a SiteMinder Admin and are asking this question, you might want to consider another job... SiteMinder is not something you can quickly wrap your head around because there are many components that make up a SiteMinder deployment (Policy Server, Policy Store, Web Agent, Secure Access Gateway, Federation, etc...).
Either way, it is probably a good idea to take a look at the SiteMinder/Single Sign On documentation (there is a lot of it). Most of the documentation is not really geared towards people new to SiteMinder, but the topics covered should provide an idea of the types of questions to expect.
There is also a free CA Single Sign On/Federation Overview course (webcast).
CA Single Sign-On & CA Federation r12.52: Overview 100 (04SMR10190) - CA Technologies