Apple calls it "ad hoc distribution."
There are many guides, for example: http://bluxte.net/musings/2009/05/17/ad-hoc-distribution-iphone-application . As Marcelo said, you will need udid from the ipad - you can see it on itunes, connect the device and click the "serial number" field in the device summary.
It is much easier to use the latest versions of xcode - after you have the sorted certificates, there is a "create and share" function in xcode that will build the necessary .ipa file and automatically add it to the email. To do this, use the "build and archive" entry "in the" build "menu, then in the organizer window, select" archive applications ", then the date you just created, then" share application ... ", then 'email ...'.