My extremely brief answer would be it is the study of how medical data is recorded, stored, retrieved, displayed and exchanged. One of the big problems facing medicine is how to transfer medical data from one system to another.
Another answer comes from the AMIA.ORG site:
Medical informatics has to do with all aspects of understanding and promoting the effective organization, analysis, management, and use of information in health care. While the field of medical informatics shares the general scope of these interests with some other health care specialties and disciplines, medical informatics has developed its own areas of emphasis and approaches that have set it apart from other disciplines and specialties. For one, a common thread through medical informatics has been the emphasis on technology as an integral tool to help organize, analyze, manage, and use information. In addition, as professionals involved at the intersection of information and technology and health care, those in medical informatics have historically tended to be engaged in the research, development, and evaluation side of things, and in studying and teaching the theoretical and methodological underpinnings of data applications in health care. However, today medical informatics also counts among its profession many whose activities are focused on dimensions that include the administration and everyday collection and use of information in health care. For an account of some of the definitions that describe the field in more formal and scientific terms, click here.
The Good Electronic Health Record (GEHR) , a major part of the work of the openEHR Foundation, is an evolving electronic health record architecture designed to be comprehensive, portable and medico-legally robust. It has been developed from the Good European Health Record project requirements statement and object model- the most comprehensive requirements documents ever developed for the electronic health record. This website is a public resource for documents and resources that have been used to build implementations of this record.
HL7lib Health Level 7 Library is a project that will provide a free, correct implementation of Health Level 7 functions. Health Level 7 is commonly used in large hospitals to send patient information among computer systems from different vendors. Since there is no reference implementation of HL7 many of these vendor systems vary widely in their interpretation of HL7.