It is the SQL extension called analytics. The "over" in the select statement tells oracle that the function is a analytical function, not a group by function. The advantage to using analytics is that you can collect sums, counts, and a lot more with just one pass through of the data instead of looping through the data with sub selects or worse, PL/SQL.
It does look confusing at first but this will be second nature quickly. No one explains it better then Tom Kyte. So the link above is great.
Of course, reading the documentation is a must.