Governor limits are limits enforced by the Apex runtime engine to ensure that runaway Apex does not monopolize shared resources in the multitenant environment. These limits are tracked and enforced during the execution of Apex code and if a governor limit is exceeded an uncatchable runtime exception is generated, such as "Too Many SOQL Queries".
Questions related to avoiding/breaching/handling Governor Limits should have this tag.
Apex has Limits Methods which provide a way to get information about governor limits at runtime.
See Understanding Execution Governors and Limits and Governors in Apex Code for an introduction to and to learn more about governor limits.
