A business plan is an internal document for how you want to sustain or grow your business. A business proposal is an external sales document used to create new business.
Your business plan should firstly set out the goals and mission for your company - usually for the following twelve months. Update it every three months or so. It doesn't have to be a long document, but give some thoughts to realistic targets and how you are going to achieve them. Consider any risks or threats and what you can do to counter them.
A business proposal is for a client. What is their business need and what are you going to do to provide a solution that meets that need. Persuade the client by showing providing a solution and backing it up with evidence that you can deliver. Solution + credibility + value = persuasion. Be credible in your price as well as reputation.
I could write all night about this, but these are some very quick thoughts. There is sometimes confusion between a plan and a proposal when the business plan is used to try and persuade potential investors in your company. However, it is still an internal document.
