There are no standards in this area. Authors and theorists make it up as they go. The entity-relationship model (ER) has hundreds of derivitives (bachman, chen, ibm, IDEF1x etc.). the most popular of the OO models is Unified Modeling Language (UML). Actually UML and IDEF1x are closest to becoming a standard that can support software products. Rational already has products and IDEF1x is the language of ERwin.
Don't be fooled by these variations. They all represent the same things, you have to be very careful that you understand all of the non-standard symbols or you will surely make mistakes in interpreting what the pictures mean.