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Strings are immutable, indexable, and use double quotes. Triple-quoted raw strings ("""...""") preserve newlines and don't process escapes.

val s = "hello"
println(s.length)               // 5
println(s.uppercase())          // HELLO
println(s + ", world")          // hello, world
println(s.contains("ell"))      // true

String templates embed expressions with $ (or ${expr} for non-trivial expressions):

val name = "Alice"
val age = 30
println("$name is $age years old")
println("Next year: ${age + 1}")

Most things you'd reach for String.format for can be done with templates. Kotlin's standard library has a huge String API — see kotlin.text for split, substringAfter, padEnd, etc.