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 crashing/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 a feature just like feature detection, but uses another function because it assumes it will also exist, e.g.:

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

This is not really recommended. Feature detection is more foolproof.

UA String

This is a browser-reported string that allows the network protocol peers to identify the application type, operating system, software vendor or software version of the requesting software user agent. It can be accessed via navigator.userAgent. However, the string is tricky to parse and can be spoofed. For example, Chrome reports both as Chrome and Safari. So to detect Safari you have to check for the Safari string and the absence of the Chrome string. Avoid this method.