Scikit-learn is a general-purpose machine learning library is better for traditional Machine Learning, while TensorFlow (tf) is positioned as a deep learning library is better for Deep Learning. The obvious and main difference is that TensorFlow does not provide the methods for a powerful feature engineering as sklearn such as dimensional compression, feature selection, etc.
If you are interested to learn Machine Learning and implement ML projects, check out this Machine Learning Online Course by Intellipaat.
Also, watch this video on Machine learning with Scikit-Learn to get started:
