Security Information and Event Management (SIEM) is a Gartner coined term to describe solutions which monitor and help manage user and service privileges, directory services, and other system configuration changes in addition to providing log auditing, and review and incident response.
SIEM differs from Log Management, which refers to solutions which deal with large volumes of computer-generated log messages (also known as audit records, event-logs, etc.)
Log management is aimed at general system troubleshooting or incident response support. The focus is on collecting all logs for various reasons. This “input-driven” approach tries to get every possible bit of data.
This model fails with SIEM-focused solutions. Opening the floodgates, admitting any/all log data into the tool first, then considering what (if any) use is there for the data, reduces tool performance as it struggles to cope with the flood. More preferable is an “output-driven” model where data is admitted if and only if its usage is defined. This use can be defined to include alerts, dashboards, reports, behavior profiling, threat analysis, etc..
Buying a SIEM solution and using it as log management tool is a waste of money. Forcing a log management solution to act like a SIEM is folly.