LLMs (Large Language Models) are artificial intelligence systems that use deep learning techniques to generate human-like text, understand and interpret natural language inputs, and perform a variety of complex tasks such as machine translation, summarization, question answering, etc. They are pre-trained on large datasets and can be fine-tuned for specific tasks or domains. The most commonly used LLMs include GPT-4 by OpenAI, BERT by Google, Bloom by BigScience, Llama by Meta, among others. When choosing the best LLM for a particular use case, consider factors like primary task, model size, pre-training level, accuracy, integrations, scalability and cost.