After logging the system, you will see 4 main areas: the home tab, the menu, the favourites section and the recent items.  

The menu is a drop down which contains the hundreds of windows, processes, reports and form you can work with in iDempiere.  If you already know which module you are looking for, then you can just type the name on the search field and it will filter down the modules to the ones closest to what you typed.

The favourites section is a way for the user to pin all the modules he regularly uses.  This provides a quick way for him to open a module without going through the menu nor the search button. On the other hand, the Recent Items contains all the recently opened items a user worked on.  

The home tab contains general information you will need from time to time.  It can be configured by the user to suit his requirement. Widgets such as calendars, running jobs etc. can be added or removed from this tab.  The upper left hand of the home tab contains specific information for the user. The Notice tab contains general information for system processes.  Requests contain specific requests that have been addressed to the user. The Workflow Activities is a quick way to determine if there is a specific action that the user needs to do.  This can be used as a means for approvers to know if there are any documents such as requisitions that need approval. Finally, unprocessed documents contain all documents whose status have not been completed.