legal main method signature in java

class NewClass{
    public static void main(String a){
        System.out.print("Hello");
    }
}

When I'm trying to execute above code, then it shows an error: main method not found. When I changed the signature to

public static void main(String... a)

or

public static void main(String a[])

Then it works. My question is how many different ways we can write legal main method signature? And what does the String... version mean?