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A base model just continues text; an instruct model has been post-trained to follow instructions and chat.
The base (or 'pretrained') model is the raw next-token predictor — feed it 'The capital of France is' and it answers, but ask it a question and it might just generate more questions. The instruct (or 'chat') variant has been through SFT and RLHF/DPO so it actually obeys prompts, holds a conversation, and respects a system prompt. As an app builder you almost always want the instruct model; base models are mainly for researchers and for doing your own fine-tuning from scratch.
Plainly
Think of Base vs Instruct Model as the brain part that guesses or decides. A base model just continues text; an instruct model has been post-trained to follow instructions and chat.
In practice
Use it when model choice, prompts, latency, cost, or quality affect the product result. In practice, define the owner, input, output, and failure mode before you rely on it.