i grew up riding arabs and now i have one of my own. arabs are a very distinct breed. you can tell by their high tail carriage and their slender frames. like CrazyChester said, they do have one less vertabrae in their back although not all arabians have the dished faces. thats common for egyptian bred arabs, but not for other strains like the polish bred horses (see my horse for a reference to a polish bred arab). their heads are smaller than most other horses though. they also have a habit of arching their necks and a lot of the time they have longer necks.
arabs are very strong. they were bred to run long distances in desert climates. they're a kind breed, but very smart and opinionated (arabs get bad rap from a lot of the horse world for this) so you have to be careful to getting training in early and patience is a must. otherwise, its easy to have an arab get out of control. they want their rider to be their partner, not just someone telling them what to do. i love Rocky bc he is extremely smart and can figure a lot of things out for himself. arabs are also very sensitive horses so any mistreatment can result in deep trust issues and behavioral problems. they're excellent edurance horses as well! i hope this helps. every horse is different and a few may not conform to what i've said, but these are the traits i've observed from working with them for so many years.
They have wonderful personalities! Most never consider themselves "horses" they love being apart of the family. And like riddlesdarkangel5 said they are very sensitive to the humans in their lives. Not all have dished faces. Most arabs that I have met are very hot (energetic). My mares 28 and STILL acts like a 10 year old.
They are very loving compared to other horses, other horses will love you, but not as unconditionally. They are very strong, and a good one, has a very sharp featured body not all round like the Quarter horse. The arab has a sharply dished face, and large eyes and nostrils.
Their ears turn in slightly at the tips, they have very dark eyes, a floating trot. With dark skin unless it is under white markings. They are uber athletic!!!! With a look of eagles about them, almost like they have a secret. I in 30 years of riding never met one that was mean they really like people after all the Bedouin people would take the mares into thier tents at night they are a very loyal breed. When properly trained they can excel at any disipline of riding.