Security Information and Event Management (SIEM) is a composite of Security Information Management (SIM) and Security Event Management (SEM). Although distinctions between these three concepts are not well defined, in general SIEM and related systems strive to automate both the analytic and the response processes for security-significant occurrences on networks.
Some common characteristics of SIEM include:
SIEM is generically similar to Intrusion Detection Systems (IDS) or Instruction Prevention Systems (IPS), although with SIEM there is a heavier emphasis on analytics and reporting.
References:
http://www.wernerschmidt.com/blog/index_files/6bf90acaf61fb4b4a614a6453df16439-16.html