A Letter of Intent is a written document that outlines a preliminary agreement between two parties regarding the terms of a potential purchase or other transaction. The two parties can settle on certain terms while agreeing to continue to negotiate the other terms and details of the transaction before actually signing a purchase agreement.
Without job-level specifics, it can be tempting to get stuck in an "I'm awesome" loop without giving enough specifics. If you have a general idea of what job groove you'd be seeking at this particular company, build your letter body around that. If you're truly just trying to get a foot in the door at a company, you can use clues from your research about the company. What does their website say about their mission and priorities? What do current and past employees say about the company in online chatter? Even without specifics, you can cobble together a pretty good idea of what the company is seeking in potential employees. Sell your skills and experience points that are special to you, and especially relevant to your industry.