Textual entailment is the process of determining if a source T implies the hypothesis text H. Example :It is a unidirectional relationship : text: If you help the needy, God will reward you. hypothesis: Giving money to a poor man has good consequences.
Some techniques for textual entailment include lexical similarity based techniques to identify if components of hypothesis are subsumed lexically in the source text, classifier based approaches gathering features from source and target text in terms of textual overlap, embedding distance and so on.
Paraphrase : “X wrote Y” is a paraphrase of “Y was written by X”. They are semantically equivalent, it is a bidirectional relationship. Techniques for paraphrase detection are similar to textual entailment except that we are looking at a bi-directional relationship instead of a unidirectional one.
Both textual entailment and paraphrase detection are widely used in many applications such as question answering, summarization and machine translation.