What's the difference between feature detection, feature inference, and using the UA string?

Feature detection

Feature detection involves working out whether a browser supports a certain block of code and running different code depending on whether it does (or doesn't), so that the browser can always provide a working experience rather than crashing or erroring in some browsers. For example:

if ('geolocation' in navigator) {
  // Can use navigator.geolocation
} else {
  // Handle lack of feature
}

Modernizr is a great library to handle feature detection.

Feature inference

Feature inference checks for one feature just like feature detection, but then uses a different function, assuming that function will also exist. For example:

if (document.getElementsByTagName) {
  element = document.getElementById(id);
}

This is not recommended. Feature detection is more foolproof.

UA string

This is a browser-reported string that allows network protocol peers to identify the application type, operating system, software vendor, or software version of the requesting user agent. It can be accessed via navigator.userAgent.