Altmetrics (or more precisely "altmetric indicators") measure scholarly influence by tracking activity in online tools and environments, including social reference managers like Mendeley, microblogging platforms like Twitter, and even article commenting systems like this one. And yes, as you suggest, that includes citation in Wikipedia, too.

Although the name "altmetrics" has provoked some unfortunate and understandable confusion, it does reflect that these new traces of scholarly influence present a potentially valuable alternative to traditional bibliometric approaches.

These traditional approaches, largely built on analyzing formal citation networks, remain and will remain valuable. Altmetrics are not a replacement for citation analysis. But there are questions citation analysis is ill-suited to answer:
* How do we track *informal* influence or "scientific street cred" (to use Blaise Cronin's memorable phrase)?
* What about impact of scholarly products lacking robust citation norms, like software, data, and blog posts?
* How do we track impact on audiences who don't cite: practitioners, clinicians, policy-makers, and the general public?

Altmetrics may help us answer these kinds of questions, and so provide an important complement to traditional citation analysis. They're a potentially powerful addition to the scientometric toolkit.

That said, citation-based bibliometrics has a half-century of research and theory behind it, while altmetrics is still a very young and relatively unexplored area. The identity of this community is still evolving, and I'd encourage folks to engage with the broader literature for a more complete answer...the above is all just one guy's opinion. Check out the upcoming http://altmetrics.org/alt... workshop, for example...great organizers, and looking forward to lots of cool research being presented.

Hope that helps...thanks for the question! Apologies for the lack of citations; not sure what norms are for PLOS question answers, and assuming "not much citation" because I'm short on time :)
j