There is a small, but important difference. In a proposal, you declare what the problem, question or argument is, and what research you to plan to do in order to answer the question or support your supposition. It can be short or several pages long, depending on how detailed you are with your research plan and the points you hope to cover.
An abstract is a condensation of your essay or paper, giving both the problem/question, and the gist of your findings, and is generally from a paragraph to a single page long.
In the event you are asked for an abstract before you have written the paper, then you state the problem and your anticipated solution.