The Institute of Medicine lists crucial core server elements for an EHR system. These include:
Health information and data storage component - that would be a database or a series of databases.
Results management, which is, essentially, software that actively manages the results - particularly lab & radiology results - that come into the EHR to assure that they are seen & dealt with appropriately by the clinician.
Decision support, which is a computer logic that presents information to help clinicians make correct decisions, such as displaying relevant reference information on the screen while orders are written or popping up warnings if a drug order appears inappropriate based on a patient's known allergies.
Electronic communication and connectivity software is not included, because of security issues.