ChatGPT (Generative Pre-trained Transformer) is a conversational AI chatbot created by OpenAI, a research organization located in San Francisco, California. It was first made available to the public in November 2022 and is based on the GPT language model architecture, which had seen several iterations before its widespread release. The launch of GPT-3.5 marked a significant milestone, allowing non-technical users to interact with a language model at no cost. The application gained 1 million users within just five days of its launch. The conversation began long before, with ELIZA in 1966, the first program people willingly talked to, and ChatGPT is the point where that line finally reached the world.

At its core, ChatGPT uses a deep learning model called a Transformer, which was first introduced by Google in 2017. The primary aim in developing this model was to create a tool capable of understanding and generating human-like text. Its applications are varied, including assisting with writing and research, answering questions, solving problems, and supporting learning across numerous subjects.

In 2018, OpenAI published the GPT paper, which described a model with 117 million parameters trained on the BooksCorpus dataset, showcasing the potential of transformers for open-ended language generation. By February 2019, GPT-2 had been released with 1.5 billion parameters, capable of writing entire essays and stories and answering questions convincingly. In 2020, GPT-3 emerged, demonstrating the ability to mimic writing styles, write code, translate languages, and perform basic reasoning, though it was initially available only via API for developers. In 2022, OpenAI enhanced GPT-3 using human feedback to improve its adherence to instructions, and GPT-4 was introduced in 2023 for ChatGPT Plus users.

In 2024, OpenAI began implementing persistent memory, enabling ChatGPT to retain information about users across multiple conversations. Additionally, it introduced multimodal capabilities, enabling the understanding and generation of both text and images.

The most recent version of ChatGPT, as of 2025, is GPT-4.5, which was released in February 2025.