Quick answer
Later means at a subsequent time — see you later. Latter means the second of two things already mentioned: "tea or coffee — I prefer the latter." Latter always needs two items, and it pairs with former for the first.
The core difference
One is about time; the other is about position in a list of exactly two.
- later — subsequently: a later edition, later that evening.
- latter — the second of two: the former … the latter.
The two-item rule
Latter strictly refers to the second of two. With three or more, the correct word is last — "the last of these", not "the latter". Using latter for the final item in a longer list is a common slip.
The other latter
Frequently asked questions
Can I use latter with three items?
Strictly no. Latter refers to the second of two; with three or more use "the last".
What is the opposite of latter?
Former, for the first of two things mentioned.
Does latter always need a pair?
In the "second of two" sense, yes. But "the latter half of the century" uses a separate sense meaning late in a period.