Helvetica comes on Macs and Arial comes on PCs, but that’s not the end of what divides them. Arial was acutally designed in-house at Microsoft to have the same spacing as Helvetica; basically, this means that if you took a 22 ½ page document in Helvetica, and changed it to Arial, it would still be exactly 22 ½ pages.
Helvetica is characterized by horizontal and vertical cuts on its letters, Arial lacks these. There are a few other smaller differences, but they’re really hard to see with the naked eye.
Most designers prefer Helvetica. To be honest, though, I think there are far better options than Helvetica, too.
I’m a huge fan of Akzidenz-Grotesk, for example.