Weight of a link typically depends on authority of the page where it is installed. In turn, authority of this reference page depends on the number and property of links to it. Authority of a page for each search engine is defined differently. For example, for the Google this is PageRank, which has values from 0 to 10, is seen in the Google toolbar and Google catalog.
For Yandex this is the quotation index, which is seen in a Yandex catalog and with the help of a special request (http://bar-navig.yandex.ru/u?ver=2&lang=1049&url=http://www.site.ru/&show=1&thc=0).
On the other hand, weight of an outbound link directly depends on the number of links in a page, its position, layout, subjects.