Abstracts (what did happen in the past) briefly summarize the highlights of finished articles and projects.
Proposals (what will happen in the future) are used to submit project ideas, oral presentations, completed papers, and posters to be considered for acceptance in academic publications, conferences or for grants. Proposals tend to be longer, usually between 350-500 words, and detail what the researcher plans to do.
Science and Social Science proposals follow the same model as the traditional research paper: Introduction; Literature Review (this will most commonly be a brief mention of what particular theory will be applied to the project versus listing multiple other researchers as in a completed paper); Methodology; Results (anticipated); Conclusion. Although proposals follow this structure, they are submitted as normal paragraphs and do not have interior section titles, such as "Introduction."
Humanities proposals, like any good story, contain a beginning (why this topic should be explored), a middle (how you intend to do it and what you will be looking for), and an end (why it will matter).
A well-written proposal will "sell" others on the project, stating not only the facts but why it is interesting, and why other professionals in the field would benefit from hearing or reading about it. After the research is complete, the proposal can easily be edited to become the abstract, with updates as to what actually happened after the research was performed.