So far, these have been quite elusive concepts to me. I had a vague idea of what they were and how they worked, but could definitely not put that into words. Vance's post on this topic on the Multiliteracies YG has clarified this and I would like to quote him here (hope this is OK)
In our course on Multiliteracies for Social Networking and Collaborative Learning environments, we intend to get into the concept of e-Portfolios, the idea being that anyone undertaking a learning journey can record and document the stages of that journey at a central hub with links to the associated documents.It's a portfolio in the sense that it can be viewed as a collection of documents in one container, but in fact what appears to be the container is a page linking to documents that could in fact be anywhere. The central page is where you send people if you want them to see what you have learned or accomplished. This could also be seen as a kind of CV professionally, or as an assessment tool at the end of a course of study for students.
Regarding aggregation, you can think of RSS and tagging on one side and aggregation as being the other side of the same coin. RSS is code that announces when a content on a site has been updated. Tags are keywords millions of users give to that content when they either produce or see it (because of it's grass roots nature, a tagging system is called a folksonomy).
Aggregators are tools that do two things. They read RSS code (when they have been given the URL of that code, or subscribed to it). And they search places where tags are likely to be found and they return links to content having the tags requested.
So aggregation is a kind of search that works off METAdata; that is code or tags or script associated with content, not the content itself. Google on the other hand searches actual content.