A mashup is a website or application that combines content from more than one source into an integrated experience. They were once something that geeks pulled together typically overlaying information onto a map or pulling together two information sources in some interesting way. In the past you needed tob e a pretty good programmer to create and publish a mashup.
Microsoft, Google and Yahoo have all nearly simultaneously released mashup creators which are accessible to the the average users.
The ease with which you can now create a mashup and how its slowly gaining acceptance in the non-programmer community is similar to the way in which websites were developed. It used to be that you needed to be an experienced web-developer to create your own website but today almost anybody can create their own using WYSIWYG tools.