A system thread is created by the JVM and runs in the background of the application. For example, the garbage-collection thread is a system thread that is created by the JVM and runs in the background, helping to free memory that is no longer in use. For the most part, the execution of system-defined threads is invisible to the application developer. When a system-defined thread encounters a problem and cannot recover, such as running out of memory, it generates a Java Error, as opposed to an Exception .