Tail recursion is a specific form of recursion where the recursive step is the last step in a function.
(This is ignoring any actual language/compiler semantics, laziness or not1)
factorial :: Int -> Int
factorial n
| n == 1 = 1
| otherwise = n * factorial (n - 1)This evaluates factorial 5 as
factorial 5
5 * factorial 4
5 * 4 * factorial 3
5 * 4 * 3 * factorial 2
5 * 4 * 3 * 2 * factorial 1
5 * 4 * 3 * 2 * 1
5 * 4 * 3 * 2
5 * 4 * 6
5 * 24
120
factorial :: Int -> Int
factorial n = go n 1
where
go 1 x = x
go n x = go (n - 1) (n * x)This evaluates factorial 5 as
factorial 5
go 5 1
go 4 5
go 3 20
go 2 60
go 1 120
120
Basically, the part of the last step in the non-tail recursive version can be shoved into a running total inside of the top-level function.