JavaScript - Help To Wannabe Js Programmer... Please

window.onload = initForms;

function initForms() {
	
	for (var i=0; i< document.forms.length; i++) {
		document.forms[i].onsubmit = function() {return validForm();} // I don't understant the syntax on the right side the the '='... Why do we need 'function ()'... why not '= validForm() instead? 	
	

	}
}


function validForm() {
	var allGood = true;
	var allTags = document.getElementsByTagName("*");
	

	for (var i=0; i<allTags.length; i++) {
		
		if (!validTag(allTags[i])) {
			allGood = false;
		
		}
	}
	return allGood;


 // from that point on, I am having a difficult time understanding the relations between the 2 functions 	

	function validTag(thisTag) {
		var outClass = "";
		var allClasses = thisTag.className.split(" ");
	
		for (var j=0; j<allClasses.length; j++) {
			outClass += validBasedOnClass(allClasses[j]) + " ";
			
		}
	
		thisTag.className = outClass;
	
		if (outClass.indexOf("invalid") > -1) {
			thisTag.focus();
			if (thisTag.nodeName == "INPUT") {
				thisTag.select();
			}
			return false;
		}
		return true;
		
		function validBasedOnClass(thisClass) {
			var classBack = "";
		
			switch(thisClass) {
				case "":
				case "invalid":
					break;
				case "reqd":
					if (allGood && thisTag.value == "") {
						classBack = "invalid ";
					}
					classBack += thisClass;
					break;
				case "radio":				
				case "email":
					classBack += thisClass;
					break;
				default:
					if (allGood && !crossCheck(thisTag,thisClass)) {
						classBack = "invalid ";
					}
					classBack += thisClass;
			}
			return classBack;
		}
				
		function crossCheck(inTag,otherFieldID) {
			if (!document.getElementById(otherFieldID)) {
				return false;
			}
			return (inTag.value == document.getElementById(otherFieldID).value);
		}
	}
}