I always thought that Cambions (only considering Infernal Cambions here for simplicity) were half devils, whereas Tieflings were more distantly related to devils.
From the PHB, pg. 42:
Tieflings are derived from human bloodlines, and in the broadest possible sense, they still look human. However, their infernal heritage has left a clear imprint on their appearance.
And
… tieflings know that this is because a pact struck generations ago infused the essence of Asmodeus—overlord of the Nine Hells—into their bloodline.
And from the MM, pg. 36:
A cambion is the offspring of a fiend … and a humanoid (usually a human).
But then from Xanathar's Guide to Everything, we have this table on page 62:
Tiefling Parents
d8 Parents
1-4 Both parents were humans, their infernal heritage dormant until you came along.
5-6 One parent was a tiefling and the other was a human.
7 One parent was a tiefling and the other was a devil.
8 One parent was a human and the other was a devil.
These last two entries on this table are what confuse me. So apparently a tiefling can be a half devil as well? Then what makes it different from a cambion? Is there a difference? Is a cambion basically just a "monster manual" tiefling as opposed to a playable race tiefling?
Note that I'm chiefly interested in 5e, but if information from older editions is required to explain this, I'll accept that information too, but I'd prefer 5e information.
Best Answer
XGtE is optional
It's important to remember that most of XGtE's content comprises optional rules, and this particular section on character generation is no exception. In fact, the opening on page 61 explicitly states:
Stretching that a little, we could infer it to mean that despite what might be implied by the options presented here, none of it should be taken as informing the actual rules of the game. It definitely doesn't help that this section of Xanathar's does contradict the racial history given for tieflings in their own description in the PHB. But if we want to take the content as presented and try and make it work, we could consider the variability of genetic expression.
All cambions are half-fiends (but not all half-fiends are cambions)
Simply put, though a cambion is a half-fiend and is always the result of union between a fiend and a humanoid, the union of a fiend and a humanoid does not always result in a cambion. Sometimes, when a devil and a humanoid procreate, perhaps the result looks more like a tiefling.
To draw a crude analogy to the real world, children of mixed race parents can vary wildly between strongly expressing the racial characteristics of one or the other parent or appearing somewhere in between - depending on exactly what random bits of genetics get passed on and in which combination. It's not even unusual for children from the same parents to appear to be completely different races!
Obviously the genetics of a fantasy world are much more complicated than ours could conceivably be (especially when you allow for magical factors). It is not much of a stretch to imagine that the children of a devil/humanoid pairing may sometimes express much more of their fiendish heritage (producing a cambion) and sometimes less (producing a tiefling).
What's the difference?
The general difference between a tiefling and a cambion is that a cambion is a very strong expression of fiendish ancestry that perhaps you can only get with one directly fiendish parent, but a tiefling is a much lesser expression of such ancestry which can also occur in descendants far removed from the original fiendish influence.