English question, please help?

A janitor or custodian is responsible for a building's routine or daily maintenance (its operating systems, cleanliness, etc) whereas a security guard, as the name implies, functions somewhat like an on-site policeman who monitors all building activities (people entering, deliveries,etc) to be sure the environment is safe and secure.

A security guard is on a job for the safety of the people who work there. He observes people's comings and goings, but a lot time is an unarmed person who is sent from a company for hire to be some sort of watchman.
Some places may employ guards 24 hours a day.
A janitor is in a building to take care of cleaning such as sanitizing restrooms and supplying them with paper, and sweeping and mopping floors,etc.