So...what's the difference between an index and a key?
Just kidding. 'Sides, I'm assuming it's what you'd think: the index is kind of a list of contents and the key is...well...a list of what = what, kinda.
At least today I can officially say I've lost four pounds since the beginning of the year. Less than a pound a week, so I'm not speedy, but it's something. If I don't welch out, by late spring I might actually finally get to the goal I tried for three years ago.
Index is the order an item appears in the list: the first item, second item, third item, etcetera. Key is some value the user sets which is unique across the list and identifies the item in some way. Like a user ID or bar code. The confusion is that in C#, sorted lists have keys. But you can't directly say "SortedList.GetByKey". You have to say SortedList.GetByIndex(IndexOfKey). Which is confusing I admit but....