|PARTITION BY||The field(s) that follows PARTITION BY is the one that the 'grouping' will be based on|
The OVER clause determines a windows or a subset of row within a query result set. A window function can be applied to set and compute a value for each row in the set. The OVER clause can be used with:
so someone can compute aggregated values such as moving averages, cumulative aggregates, running totals, or a top N per group results.
In a very abstract way we can say that OVER behaves like GROUP BY. However OVER is applied per field / column and not to the query as whole as GROUP BY does.
Note#1: In SQL Server 2008 (R2) ORDER BY Clause cannot be used with aggregate window functions (link).