The function counts the number of vowels (a, e, i, o, u) in the input string, considering both uppercase and lowercase vowels.

[CODE]: 
```python
def count_vowels(input_string):
    vowels = "aeiouAEIOU"
    count = 0
    for character in input_string:
        if character in vowels:
            count += 1
    return count
```

This code defines a function `count_vowels` that takes an input_string as an argument, goes through each character in the string and counts the occurrences of vowels. The function returns the count of vowels present in the input string.