Chatter is a feature of Salesforce that enables you to communicate to the entire organisation. It can be best thought of as an internal twitter for St John staff, without direct messaging. The messages you posted on Chatter are not private and any staff member can see them. It is a quick method to communicate within Salesforce.

Typically Chatter messages will occur automatically when creating or changing other objects in the system such as Cases, Leads or Opportunities.

All Chatter feeds can be viewed by the entire organisation. Staff can filter their feeds to what is applicable to them by using the different feed options.

Notifications will only appear for feeds that you are following or that you are tagged in.

With this in mind, it is essential that all communication in chatter is professional and relevant there is no privacy or expectation of such.

Using Chatter (Link)

Using chatter objects (Link)