What Is ChatGPT? How It Works and What It Can Do

What is ChatGPT?

ChatGPT — short for Chat Generative Pre-trained Transformer — is a large language model (LLM) product released by OpenAI in November 2022. It presents a chat interface on top of OpenAI's GPT family of models, allowing users to have multi-turn conversations in natural language.

Unlike a search engine that retrieves existing documents, ChatGPT generates new text token by token, predicting what word or phrase should come next based on patterns learned during training. The result feels like a conversation with a knowledgeable assistant — but that feeling can be misleading, because the model has no understanding of truth in the way humans do.

OpenAI made ChatGPT available for free research preview, and it reached one million users within five days of launch — faster than any consumer technology product in history at that point. By 2025, it had hundreds of millions of active users and had become a central tool in education, software development, research, and creative writing.

The GPT architecture

GPT stands for Generative Pre-trained Transformer. The "transformer" part refers to the neural network architecture introduced in the 2017 paper "Attention Is All You Need" by Vaswani et al. at Google. Transformers replaced older recurrent networks (RNNs) as the dominant architecture for language tasks because they process all tokens in a sequence simultaneously rather than sequentially, enabling much faster training on large datasets.

The key innovation in transformers is self-attention: a mechanism that lets the model weigh how relevant each word in a sequence is to every other word. When processing the sentence "The bank by the river was muddy," self-attention lets the model recognize that "bank" relates to "river" (not financial institutions) by attending to surrounding context.

GPT models are decoder-only transformers — they are designed specifically for text generation. The model takes a sequence of tokens (words or word fragments) as input and outputs a probability distribution over the next token. By sampling from that distribution repeatedly, the model generates coherent paragraphs, code, or any other text.

How ChatGPT is trained

ChatGPT's training involves three major stages:

1. Pre-training

The base GPT model is pre-trained on an enormous corpus of text from the internet, books, code repositories, and other sources — hundreds of billions to trillions of tokens. During pre-training, the model learns to predict the next token in a sequence. This phase installs broad language knowledge: grammar, facts, reasoning patterns, and writing styles.

2. Supervised fine-tuning (SFT)

Human trainers write example conversations — both the user prompt and the ideal assistant response. The model is fine-tuned on these examples to behave helpfully in a dialogue format, rather than simply completing text as it would after pre-training alone.

3. Reinforcement Learning from Human Feedback (RLHF)

Human raters rank multiple model responses from best to worst. A separate "reward model" is trained to predict these rankings. The GPT model is then optimized using reinforcement learning to generate responses the reward model scores highly. RLHF is what makes ChatGPT feel polite, helpful, and safer — it steers the model away from harmful outputs and toward responses users prefer.

What ChatGPT can do

ChatGPT performs well across a wide range of text-based tasks:

• Drafting and editing text — essays, emails, reports, cover letters, and creative writing.
• Summarization — condensing long documents into bullet points or paragraphs.
• Explaining concepts — breaking down complex ideas at any level of detail.
• Brainstorming — generating topic ideas, research angles, counterarguments, or outlines.
• Translation — producing reasonable translations across dozens of languages.
• Code generation and debugging — writing, explaining, and fixing code in most programming languages.
• Question answering — answering factual questions from its training data (with caveats — see below).
• Structured data tasks — extracting information, formatting tables, and classifying text.

Newer versions of ChatGPT also support image input (vision), image generation via DALL·E, web browsing, and tool use through plugins or the GPT-4o API.

What ChatGPT cannot do well

ChatGPT's most important limitation is hallucination — it generates plausible-sounding but factually incorrect content, including fake citations, invented statistics, and non-existent laws or court cases. The model does not "know" it is wrong; it simply generates the most probable next token, which is sometimes false.

• No real citations — any citations ChatGPT provides may be fabricated. Always verify in an actual database.
• Outdated knowledge — information past the training cutoff is unavailable (unless web browsing is enabled).
• Mathematical reasoning — GPT models struggle with multi-step arithmetic and complex symbolic math.
• Logical consistency — in long conversations, the model can contradict itself or lose track of earlier context.
• No genuine understanding — the model manipulates statistical patterns, not semantic meaning.
• Bias — training data reflects societal biases, which can surface in generated content.
• Confidently wrong — unlike a human expert, ChatGPT rarely expresses appropriate uncertainty.

ChatGPT versions compared

The free ChatGPT tier uses GPT-4o mini or GPT-4o; ChatGPT Plus subscribers get access to the latest frontier models. OpenAI also exposes models through an API for developers.

Using ChatGPT in academic work

ChatGPT can be a useful writing assistant for brainstorming, outlining, and getting feedback on drafts. It is not a reliable source of citations or factual claims — always verify any information it provides against primary sources. Many universities now have explicit policies on AI use; check your institution's guidelines before submitting AI-assisted work.

For tasks like improving the clarity of your own writing, CiteGenie's Writing Assistant is designed specifically for academic contexts and helps you refine your prose without replacing your ideas.