What is the difference between OTRS Help Desk and OTRS::ITSM

Question

OTRS

On the OTRS website , in the distributions section, it says:
OTRS includes the leading Open Source Help Desk software “OTRS Help Desk” and the leading Open Source ITIL (R) V3 certified IT Service Management Software “OTRS::ITSM”.
A list of Bandles (sets or modules) is also indicated:
OTRS::ITSM 3.3.7
OTRS::ITSM General Catalog 3.3.7
OTRS::ITSM ChangeManagement 3.3.7
OTRS::ITSM Configuration Management 3.3.7
OTRS::ITSM Core 3.3 .7
OTRS::ITSM IncidentProblemManagement 3.3.7
OTRS::ITSM ServiceLevelManagement 3.3.7
OTRS:.ITSM ImportExport 3.3.7
Please briefly explain the difference in functionality between OTRS Help Desk and OTRS::ITSM, as well as what the purpose of the above modules is and whether they are contained inside OTRS Help Desk 3.3.7 or OTRS::ITSM 3.3.7 or they need to be downloaded and installed separately.