OpenMRS is a flexible, modular, multi-layered system, and one of its strengths is that its platform can be used in many different configurations. As a result, "OpenMRS" can mean many things.
To implement an OpenMRS system, you typically start from an existing OpenMRS Distribution, and configure that distribution for your implementation use case. Depending on which distribution you choose, and what your use case is, this may range from configuring metadata through a user interface, to writing custom add-on modules, or your own custom user interface backed by OpenMRS REST web services. Implementations with a strong development shops may choose to build their own custom application(s) on top of the Platform.
Here is a list of known OpenMRS Distributions that are intended for use by the general public:
OpenMRS Reference Application
Maintained by the OpenMRS community, this distribution demonstrates how the platform's capabilities can be used to build an EMR. You could use this out-of-the-box as a facility EMR, but it (currently) doesn't have the complete EMR feature set.
Maintained by ThoughtWorks and the Bahmni community, Bahmni is an out-of-the-box EMR and Hospital Information System that can be configured without programming.