Collection was modified; enumeration operation may not execute - why?

I'm enumerating over a collection that implements IList, and during the enumeration I am modifying the collection. I get the error, "Collection was modified; enumeration operation may not execute."

I want to know why this error occurs when modifying a item in the collection during iteration. I've already converted my foreach loop to a for loop, but I want to know the 'details' on why this error occurs.