Below is a non-tail-recursive function to compute the sum of a list of integers.
let rec sum = function |  -> 0 | h::t -> h + (sum t)
The last operation the function performs is the addition. Thus, the function isn't tail-recursive.
Below is a tail-recursive version of the same function.
let sum l = let rec aux acc = function |  -> acc | h::t -> aux (acc+h) t in aux 0 l
aux function is tail-recursive: the last operation it performs is calling itself. As a consequence, the latter version of
sum can be used with lists of any length.