Thanks for your question.
There are quite a number of differences. Here are three of the main points:
1: Level of knowledge required by a non-technical domain expert.
With Entity Relationship Modeling,(ERM), a non-technical domain expert has to begin by learning about what the ERM symbols and diagrams mean. For example, the use of the "crows foot" symbol to show than an entity is in a "many" relationship. with another entity. Then it is necessary to understand the concept that "each entity has attributes" and so on. With ORM, the domain expert just has to understand his/her own linguistic assertions - no technical knowledge is necessary. It is the job of the ORM tool to generate a fully populated ERM diagram from the assertioins made by the domain expert.
In order to ensure that the ERM model does not contain duplicate elements, it is customary to apply a "normalisation" procedure to ensure that the ER model is in a "Normal Form". When done manually, this is a complicated and frequently error prone process. Thus, many entity-relationship modelers are reluctant to do the detailed work needed to get to 5th normal form. With ORM, the ORM tool generates a fully normalised ERM diagram (in 5NF) in a fraction of a second. This frees up the time of the modeler to focus on discussiuons that ensure that the object-role model is agreed by the domain-expert.
With ERM, a small change can result in something that was an "attribute" being changed to an "entity". Such changes require re-normalisation with all its attendant problems.
The library contains several documents that give more detail on the relationship between ERM and ORM.
Look here: http://www.ormfoundation.org/files/folders/er__orm/default.aspx
Let me know if you have any more questions.