Security is a more general term and can be used for one or more than one person. A security guard is an individual; more defined. They are security or They are security guards He/she is security or He/she is a security guard I am security or I am a security guard etc.

Typically, when referring to a public place, the person who guards it is called a security guard. Example: He works as a security guard for that company. However, there are times when you can say just security, usually meaning the entity as a whole and not just one specific person. Example: I'm calling security.