According to the Symantec Internet Security Threat Report (ISTR), 400 million new variants of malware were created in 2011, which is an average of 33 million new variants of malware a month, or an average of one million new variants a day.

It is impossible to manually analyze such a large number of sample files, so it is therefore necessary to use an automated threat analysis system to analyze sample behavior and prioritize the files that virus definitions should be created for.

By searching the Web, you can find services that execute files in a sandbox and show the behavior of those files, thus enabling you to see what a suspicious file does before you execute it on your computer.

Both systems execute the requested files in a sandbox and log system behavior.

Source: Symantec