I'll elaborate and write this as an answer. Unified Modeling Language (UML) is probably the most used specification for visualizing system architectures. It was intended to be used specifically to describe object oriented systems. There is a consortium that maintains versioned standards.
UML is not the same thing as Entity-Relationship Models. Something else you might want to look at. Sometimes you'll see a mix of the two, some UML with maybe more 'crows feet' style notations to depict cardinality.
I consider Martin Fowler an authority on these matters and recommend his writings. "UML Distilled: A Brief Guide to the Standard Object Modeling Language (3rd Edition)".
Another book on the subject I can recommend is "Advanced Object-Oriented Analysis and Design using UML", by James J. Odell.
There should be many tutorials on the web too.
A great free tool for modeling is Dia. Visio of course supports UML and ERs and many others. More can be found here: http://martinfowler.com/bliki/UmlSketchingTools.html