What is it
Scikit-learn is a data science toolbox for Python with built-in machine learning algorithms such as regression, clustering and classification models. It's a flexible framework that lets you set up pipelines of typical data analysis steps such as: data ingestion, data cleaning, model fitting & prediction.
When to use it
Scikit-learn is used for exploratory analysis of static datasets for ad-hoc insights due to its simplicity. Capability to use in production envrionments is yet to be explored.
How to learn it
There are tons of tutorials for scikit-learn, since it's one of (if not) the biggest machine learning toolbox for Python. One can first of read the User guide and follow their tutorial on their website.
Platforms for online courses:
Make effective use of Scikit-learn also includes learning NumPy and Pandas.