Modeling is an art to visualize any process. Software modeling helps the engineers to understand the functionality of the system. A modeling language is any artificial language that can be used to express information or knowledge or systems in a structure that is defined by a consistent set of rules. The rules are used for interpretation of the meaning of components in the structure. A modeling language can be graphical or textual.
Graphical modeling languages use a diagram technique with named symbols that represent concepts and lines that connect the symbols and that represent relationships and various other graphical annotations to represent constraints.
Textual modeling languages typically use standardized keywords accompanied by parameters to make computer-interpretable expressions.
In these days UML is the most popular modeling language.

Thanks
Gurjinder Singh Brar
http://dotnet-question-answer.blogspot.com