Wikimetrics is a web application. There are three concepts you, the user, should be aware of:
A cohort is a list of users. All users can belong to the same wiki project or each can specify a different project. Cohorts can be defined in CSV files and uploaded here. Examples of useful cohorts might be: Wikipedia editors that participated in an outreach event, Wikimedia Commons users that are also active on other Wikis, and so on.
A metric is a measure of some type of on-wiki activity. Metrics are easy to implement if you develop in Python or if you can show that a new type of measurement might be useful. In the latter case, either the analytics team or community members are likely to help you implement a new metric. To contribute code, please have a look at our repository.
A report is a way for you to ask for the results of a metric applied to a cohort. The reports are run asynchronously on a Celery queue and the results show up when they're ready, on the reports page.
You can access Wikimetrics by logging in and authenticating using your Wikipedia/Wikimedia account or a Google account. Wikimetrics welcomes all. :)