Altmetrics and traditional metrics measure different kinds of activities:
We are all familiar with the current practice of determining the value of a research article by looking at the number of times it has been cited by others, and at the impact factor of the journal in which it was published. The journal impact factor measures the reputation and value of a journal as a whole rather than the merit of the individual articles within the journal.
Increasing attention is being paid to the social or public impact of research in addition to the scholarly impact. Altmetrics help in this endeavor by looking at the number of times an individual article or other research object (such as data files or computer code) has been viewed, downloaded, shared, bookmarked, mentioned in a blog, reused, and more.
Article level metrics (ALMs)
"The dissemination of research is more immediate and occurs across more channels than ever before. Article-Level Metrics are an attempt to better reflect an article's impact under these new circumstances. ALMs can incorporate shorter-term data points such as news coverage, blog posts, tweets, and Facebook likes...They can also incorporate longer-term markers such as download statistics and article comments" (Article-Level Metrics: A SPARC Primer). Article level metrics incorporate both traditional metrics and altmetrics by considering the number of citations along with numerous altmetric indicators.