These are diagrams which show database tables, the fields in each table and the relationships between tables. I've been looking on and off for a simple and free program which will display erds, and which will preferably extract the information needed automatically from a database.
Today I found such a program: IBUtils. Loading a database and displaying its structure is a bit awkward initially, but once the data is on screen, it's plain sailing.
Here is the erd for the database which I partially described the other day: