Search Engine Optimisation
This the title that will appear in search engines and the window name in the browser, this has the most impact on page rank on your actual site (disregarding external links and other factors) when blank, the product name is used as the title.
The description is placed in the META data of the product page. What this means is that the description is not rendered on the page but is available for search engines to read. Some search engines use the meta description to render with the search results. It has minimal, if any impact on the ranking of the search result.
Plain Text Description
This field is used in your data feeds if supplied. If blank, a copy of your regular description MAY be used (subject to support by the reader of the feed). Some data feeds don't deal well with HTML formatting, that is the code which determines how the text looks on a webpage. For this purpose we provide this field to enter a description in plain text without formatting.