Pedanticaly speaking a sabre must have a curved edge, whereas backswords tend to be straight (the turcael blade being the most notable exception). Backswords tend to have greater hand protection than a sabre, though this is a very sweeping statement but it does mean that one can rely on the basket while making actions in broadsword that are not a very good idea in sabre.
Another difference is that backsword method tends to use a chop or offensive action from guard-to-guard much more and fences in-the-round to make distance, whereas sabre works with the various forms of moulinet action and is primarily linear using a lunge-and-recover action to make distance.
This is fairly evident at either ends of the dateline (c16th-19th)but gets muddied in the middle somewhat where one gets "Regimental " methods used by Highland regiments which use a traditional style of basket hilt sword but in a very sabre style. As an aside things seem to be deemed "Highland" in the C19th fencing world if they use a slip of the lead leg when making any or all defensive actions.
Caveat:There are, as usual, exceptions and contradictions to have everything I have said