Aggregation is identifying a content source and sharing everything that is found, while Curation is more focused. Curation zooms in on all of that information and selects the most important and interesting pieces.
This picture is a nice visual to show the difference between Aggregation and Curation. The content would be shirts. So Aggregation would be every shirt that was found (The Whole), while Curation would focus in on specific shirts (An Organized Selection).
Two articles that helped me to understand the difference:
This Video makes the difference very clear:
To end my night, enjoy this little comic: