In an excellent article about Google map mashups, Richard Mamanus explains the basics of creating custom google maps around other info and working witht the google maps api for mashups.
How to develop a Google Maps mashup
Posted by Richard MacManus
In my previous post I noted that Google Maps has released Version 2 of its API. In this post I thought I’d talk more about how you can start developing with Google Maps. Mashups are all the rage, so if you’re a developer or just like to remix things – now’s a good time to jump in and build one!
The easiest way to get content for your mashup is to use APIs (Application Programming Interface) from companies such as Google, Amazon or eBay. That is, big companies that offer a reliable and large data set via documented API hooks into their systems. You can also use RSS from companies like craigslist.