ELIZA is an early natural language processing system that could carry on a limited conversation with a user by imitating the responses of a Rogerian psychotherapist.

University of Michigan - Ann Arbor

Chatbots are designed for unstructured conversation, and are meant to mimic natural back and forth human dialogue

Chatbots

An on-going but a helpful book resource about NLP
https://web.stanford.edu/~jurafsky/slp3/

Speech and Language Processing (3rd ed. draft) 

Rule-based chatbots are chatbots that create their conversations from set rules, such as responding "What makes you think I _____ you" in response to the statement "You _____ me." Early notable rule-based chatbots include ELIZA, which essentially acted as a therapist to clinical psychology patients, and PARRY, which modeled schizophrenic behavior using responses based on variables for fear and anger.

Rule-Based Chatbots

Corpus-based chatbots are chatbots that draw their conversational data from corpuses, or vast linguistic datasets, of human conversation from various sources like social media and TV shows. 

Corpus-Based Chatbots

Chatbots are typically evaluated by humans, who assign the chatbot a score that weighs different characteristics of performance such as engagingness,  fluency, humanness, etc. Chatbots can be evaluated by either a user (participant evaluation) or a third-party observer (observer evaluation).

Evaluating Chatbots

Chatbot's architecture can be hybrid ,taking advantage of both rule-based and neural/corpus based systems sometimes they even utilize elements from frame-based structure.

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