"Altmetrics is the creation and study of new metrics based on the Social Web for analysing and informing scholarship." (altmetrics.org)
Altmetrics (or alternative metrics) provide article-level evidence of the societal impact of research, in terms of its mentions in social media (blogs, Facebook and Twitter), Wikipedia and other quasi-scholarly platforms, news sources and policy documents.
Altmetrics can include:
Altmetrics can help researchers understand how their outputs are being shared and discussed via social media and online and may supplement the information gained from traditional indicators. It is important to note that, altmetrics are an indicator of the attention that an output has received, not necessarily the quality of the article!
You can download the Altmetric bookmarklet for your own use. Install it in your browser to get one-click article level indicators when reading an online journal article (works with any journal article that a DOI).