Journal Entries are just records of events that have happened. As well as the title and the details, the journal entries record when the event happened. This means that it is possible to see the journal entries in the day view, as well as just the titles in the week and month views as well.
You can put anything you like into the text of the details (emails, text from websites).
The start date is just the start of the event. You enter it into the form as a standard date/time
The end date is the date and time that the event finished. If you leave this blank, the system will assume that this was an event at a single point in time (in the case, the start time).
If you have child entries for a journal entry, it is not a bad idea to leave the start and end dates blank - This lets the system use the start and end dates of the children. It will set the start date as the earliest start date of the children, and the end date as the latest end date.