343 Guilty Spark is an artificial intelligence created by the Forerunner, charged with the task of monitoring Halo after the Forerunner left for reasons unknown. His nickname, the "Monitor", suits the purpose.
Throughout the game, Guilty Spark will range from annoying to downright conniving and hostile. Although he himself cannot activate Halo, he entices the Master Chief into nearly causing the destruction of all life in the universe; it is only through the actions of Cortana that you, as the Master Chief, are prevented from this.
An interesting fact is that 343 Guilty Spark cannot be damaged using any weapon; he will simply float and make sarcastic comments to the effect of "Please stop being human!" However, he will instruct the Sentinels into attacking you once his true intentions are made known.
For more information, visit HBO's Halo Story Page about 343 Guilty Spark.