See here for an example of a link table used to link dates (=dimensions) from several tables. A fact is something you want to calculate with. Like SalesAmount where you might want to calculate sum(SalesAmount) or avg(SalesAmount). In Qlikview any field (fact or not) is a dimension.
Dimension tables and fact tables are basic business intelligence concepts. Read for example the descriptions of Fact tables and Dimension tables. In Qlikview a chart can have dimensions and expressions. This has little to do with Dimension tables, but is just a term used in Qlikview in the context of charts. Think field instead of dimension and it'll probably look more familiar to you.
Link tables are used to link dimension and/or fact tables when direct links are not possible or have unpleasant effects. See this article for an explanation.
