Object-oriented modeling is an approach to modeling an application that is used at the beginning of the software life cycle when using an object-oriented approach to software development.
The software life cycle is typically divided up into stages going from abstract descriptions of the problem to designs then to code and testing and finally to deployment. Modeling is done at the beginning of the process. The reasons to model a system before writing the code are:
Communication. Users typically cannot understand programming language code. Model diagrams can be more understandable and can allow users to give developers feedback on the appropriate structure of the system. A key goal of the Object-Oriented approach is to decrease the "semantic gap" between the system and the real world, to have the system be constructed using terminology that is the same as the functions that users perform. Modeling is an essential tool to facilitate this.
Abstraction. A goal of most software methodologies is to first address "what" questions and then address "how" questions. I.e., first determine the functionality the system is to provide without consideration of implementation constraints and then consider how to take this abstract description and refine it into an implementable design and code given constraints such as technology and budget. Modeling enables this by allowing abstract descriptions of processes and objects that define their essential structure and behavior.
Object-oriented modeling is typically done via use cases and abstract definitions of the most important objects. The most common language used to do object-oriented modeling is the Object Management Group's Unified Modeling Language (UML).