What is the difference between a value type and a reference type?

With a value type, the variable holds the data itself. Assigning it or passing it to a function makes a copy, and changing the copy never affects the original. With a reference type, the variable holds a reference to an object; assignment copies the reference, so both variables point to the same object.

struct Point { var x = 0 }        // value type
final class Counter { var n = 0 } // reference type

var a = Point(); var b = a; b.x = 5      // a.x is still 0
let c = Counter(); let d = c; d.n = 5    // c.n is now 5

Per language:

• Swift: struct, enum, tuples, String and the collections are values (collections use copy-on-write, so copies are cheap until changed). class and actor are references.
• Kotlin and Java: only primitives (Int, int, Boolean…) behave as values. Everything else, including Kotlin data class and Java record, is a reference type. They get value-like behavior when they’re immutable.

Why it matters: shared mutable objects cause “action at a distance” bugs, where code in one place changes data another place is using. Values (or immutable objects) make that impossible and are safe to share between threads.