GPT (generative pre-trained transformer) refers to a series of large language models developed by OpenAI and based on the transformer architecture. The models are first pre-trained on vast amounts of text to predict the next token and then further trained for dialogue and following instructions. The first GPT model appeared in 2018; later generations underpin ChatGPT. In everyday speech, ‘GPT’ is often used for language models in general.
The transformer is a neural network architecture introduced in 2017 in the research paper “Attention Is All You Need”. At its core is the attention mechanism: each word is weighted in relation to all other words in a text, and the computation can be parallelised efficiently. Transformers underpin today's language models such as GPT, Claude and BERT and are also used for images and audio.
A large language model (LLM) is a neural network trained on very large amounts of text, enabling it to understand and generate language. It works by calculating the most likely next token, step by step. LLMs write, summarise, translate, answer questions and generate code; well-known examples include Claude, GPT, Gemini and Llama. Without access to current sources, they only know what was in their training data.
OpenAI is an AI company based in San Francisco, founded in 2015 as a non-profit organisation. Since its restructuring in October 2025, it has consisted of the non-profit OpenAI Foundation, which controls the for-profit OpenAI Group PBC (as of October 2026). OpenAI is best known for ChatGPT and the GPT language models, as well as the DALL·E image generator and the Whisper speech recognition model. Its models are offered through apps and an API.
ChatGPT is an AI assistant from the US company OpenAI, released in November 2022, which brought generative AI to a broad audience. It is based on large language models from the GPT series and answers questions, writes and edits texts, summarises documents and helps with programming. ChatGPT is available in the browser and as an app, in free and paid tiers as well as versions for teams and enterprises.
