Bretons are more into magic than Imperials. Imperials also have the "Voice of the Emporer" power, which calms enemies, while Bretons are resistant to magic and can absorb it temporarily. Imperials are also the main people leading the Empire, since it's based out of Cyrodiil. Both races are descendants of the Nedes, although I believe the Bretons have some elven blood in them. However, other than that, I don't think there are too many other differences.
Let's list off the obvious things first. Foremost, as opposed to Imperials, Bretons have both a higher acceptence of magic and Elves, along with having some natural gifts in it. Secondly, many Bretons, based on their bloodline, will show slight Elvish features in their physical appearance, going so far as to have slightly pointed ears. You can actually see this in Skyrim and ESO especially if you look closely. Bretons also tend to have a more agile and frail frame, much like Elves, along with higher cheekbones, lean eyebrows and a thinner, more triangular jaw. Bretons are also said to have a high intelligence who enjoy pursuing either an adventerous 'knightly' errand life, performing tasks in exchange for noble status and wealth, or a scholarly life referring to education, science, the arts, etc. Their arcitechture is typically a blend of Altmeri and Nedic, showing simple stonework yet keeping in line with pointed 'Elven' arches and even some carvings said stonework. Essentially, think Solitude with a stronger Elven vibe. As opposed to the Imperials' emulation of the real-world Roman Empire, the Bretons instead opt for their warriors to be clad in full-plate armor like a classic knight of old. This is why in fan-made videos you'll often find Bretons clad in either a noble's outfit, Steel Plate armor, or a mix of the two as it befits their culture. Finally, the real-world inspiration for the Bretons turned out to be the ancient celtic Britons of England and France.
Fun Trivia: It's heavily implied by various pieces of evidence that Tiber Septim himself was a Breton, originally named Hjalti Early-Beard and born in Alcaire, High Rock.