1. Introduction  
2. Words Similarity  
2.1 Corpus Based Words Similarity  
2.2 Knowledge Based Words Similarity  
2.3 String Based Words Similarity  
2.4 Combined Methods  
3. Sentence Similarity Methods  
3.1 Word Based Sentence Similarity  
3.2 Structure Based Sentence Similarity  
3.3 Vector Based Sentences Similarity  
4. Similarity Datasets  
4.1 Word Similarity Dataset  
4.2 Sentences Similarity Datasets  
5. Conclusion