Natural Language Processing (NLP) is the field in which information is extracted from spoken or written language.
NLP is special because the meaning of a word is heavily context dependent:
The bird is a crane.
A crane is used in industry to lift heavy objects.
I have to crane my neck to look at him.
An alternative is processing the context words in addition to each target word:
This naive approach works, but is computationally intensive. Additionally, for questions of content, a few words of context is not sufficient. In the above example if asked the question "What is fun?", the model would have no way of knowing that the answer is "Deep Learning", because "fun" does not include that context.
Recurrent Neural Networks (RNN's) attempt to solve the problem of storing context by having their own output fed back into them. By doing this for each processed word, information is preserved and passed along.
This can take various forms, LSTM's being the most popular.