When is one called versus the other? Is there a situation were onChange would be called but onBlur would not be called?
onblur fires when a field loses focus, while onchange fires when that field's value changes. These events will not always occur in the same order, however.
In Firefox, tabbing out of a changed field will fire onchange then onblur, and it will normally do the same in IE. However, if you press the enter key instead of tab, in Firefox it will fire onblur then onchange, while IE will usually fire in the original order. However, I've seen cases where IE will also fire blur first, so be careful. You can't assume that either the onblur or the onchange will happen before the other one.