What is the difference between var and let in Swift?

var declares a variable whose value can be changed. let declares a constant whose value cannot be changed after initialization.

// Declaring a variable - value can change
var mutableVariable = 10
mutableVariable = 20 // Ok

// Declaring a constant - value cannot change
let immutableConstant = 30
// immutableConstant = 40 // Error: Cannot assign to 'let' value

Using let is preferable when the value is guaranteed not to change, as it increases code safety and potentially allows the compiler to perform optimizations.