As opposed to citation based metrics, altmetrics stands for "alternative metrics". While it has its differences from citation based metrics, it aims to complement them, not replace them. They are measured based upon web/digital presence. Altmetrics generally are produced by gathering information that is collected by APIs (application prorgramming interface) across the internet. An advantage of altmetrics is that data is gathered in real time and as it appears, much faster than citation based metrics. It can inform the researcher how many people have downloaded your publication, where it is being viewed, and who is sharing it, amongst other things.