Oracle EM Express is an administrative console for the Oracle 12c database and is normally installed and configured via the Database Configuration Assistant (DBCA).   It uses the internal structures of an Oracle database for monitoring and administration and SQL*Net for communication, which greatly reduces the load on database being administered.   EM Express runs against most web-browsers with Flash plug-in installed.  EM Express is not designed to administrate multiple oracle databases and natively only connects to a single oracle database.   It is part of the oracle enterprise license package, however functions like SQL tuning page will result in additional licensing charges.  EM Express is extremely useful in relieving the administrative burden of a single database installation.