From the Modelica Association home page:
The Modelica Language is a non-proprietary, object-oriented, equation based language to conveniently model complex physical systems containing, e.g., mechanical, electrical, electronic, hydraulic, thermal, control, electric power or process-oriented subcomponents.
Modelica uses a text based modeling language behind the scenes that is primarily intended for use with "patch-based" graphical wiring up of components.
There are a lot of Modelica-related tools and comparing their claimed pros and cons, capabilities, and compliance vs the Modelica Language standard is not an easy task.
Most Modelica users are at least familiar with OpenModelica (your installation and stability mileage on various operating systems can vary wildly by version).
Two of the big industrial-strength commercial players are MapleSim and Wolfram SystemModeler, which have the advantage of optionally integrating with their extremely powerful Maple and Mathematica symbolic algebra computation platforms.
Another big commercial Modelica player is Dymola from Dassault Systèmes, who have of course acquired No Magic, developers of the MD SysML Plugin, Cameo Systems Modeler®, and Cameo Simulation Toolkit®.
Just getting started with Modelica? The following comprehensive external tutorial is highly recommended: Modelica University: "Modelica by Example" by Dr. Michael M. Tiller.