Chat GPT is the most popular and well known LLM. In addition to being the open AI model that many other LLMs work off of, it also has it's own tool where users can ask a question or put in a command, and it will create a conversation to complete your task.
Chat GPT may not be the best tool for research, as it may still have some "hallucinations" (citations/articles that do not exist), and other tools (like Scite) work more closely with publishers for access to citations outside of open access. It can be helpful for brainstorming, or helping to create summaries of notes or an article that you read. It can also be helpful to get basic background information that you would then take to do your own research in the library databases.