WHAT IS FEATURE DETECTION ?
Feature Detection is Future Detection whether the new feature is available in the device / browser they are using.A new feature is like the Canvas Element Detection.
WHAT IS BROWSER DETECTION ?
Browser Detection unlike Feature detection is detection of browser version so that developers can keep some additional padding mechanism so that the nuisance creating have been handled with extra functionality or eliminate those browsers which are typically old like IE 6 which would cause their functionality to work improper.
Feature Detection versus Browser Detection
The Browser war is very fierce these days and all of them want to implement the cutting edge technology in this run all developers want to support the newest of the new technology and also support the legacy ones.