PhoneGap was originally an open source project by Nitobi for packaging HTML5 code within native ‘wrappers’ for the creation of mobile applications. Adobe acquired Nitobi and the PhoneGap technology. Adobe proceeded to contribute the PhoneGap code to the Apache Software Foundation in order to ensure that it remains open source. At this point the name was changed to Cordova, with Adobe maintaining the PhoneGap name and brand.
Adobe plans to add value to Cordova under the PhoneGap name, such as adding tool support and services such as PhoneGap Build.
Currently many people still use the PhoneGap name to describe the open source framework (possibly because it is a better name!), but over time, this will probably change as Adobe builds on the PhoneGap brand and product range.
From a developer perspective, this is a pretty good situation, the framework remains open source, but if you want added value, you pay Adobe.