1 Introduction  
2 Overview  
2.1 Definition  
2.2 Category of federated learning  
2.3 Federated transfer learning  
2.4 Dynamic heterogeneous FTL  
3 Methodology  
3.1 Homogeneous federated transfer learning  
3.2 Heterogeneous federated transfer learning  
3.3 Dynamic heterogeneous FTL  
3.4 Model Adaptive FTL  
3.5 Semi-supervised and unsupervised FTL  
4 Application  
4.1 Federated cross-domain recommendation  
4.2 Federated medical image classification  
4.3 Federated financial service  
4.4 Federated traffic flow prediction  
5 Conclusion and Future Work