'alert(this.checked)' --> the 'this' means the checkbox element/object
="doit(this.form)"> --> the 'this' means the button element and thus the form in which the button is found.
msg = (theObj.checked) ? "it's checked" : "it's not checked";
variable = (true/false conditional statement) ? ''result if condition is true" : "result if condition is false";
The first doit() function is called with onclick event handlers -- only the element concerned is being sent as a parameter so it's checked property can be tested immediately.