What are article-level metrics? Why track at this level?
Authors often track the sources that cite their work to see the impact of their research. Researchers also look for citing articles to trace the evolution of research and ideas. Just as examining an article's bibliography allows you to look backward at its influences, examining the sources that subsequently cite an article reveal its influence in its subject.
There are a number of sources available to researchers to discover citations, including Google Scholar and Web of Science.
Note: Citations are the traditional "article-level metric"--that is, a measure of the influence of published research at the article level. Most Altmetrics are also tracked at the article level; however, Altmetrics and article-level metrics are not quite synonymous.
What are the caveats and limitations?