Pastrami is heavily spiced with black pepper and other things. I think it only comes in slices (whereas roast beef is available either sliced or uncut). I don't know if there's any other differences.
Hayyim's got the basics of it down , the only thing I'd add is that pastrami is cured/smoked.What is the difference between a Slushie and a Slurpee?
Ah! Thanks, I suspected I was forgetting something.Why does water taste different at night?
Pastrami is prepared in the same way but is smoked after curing.
Roast beef (also known as a roast) is a cut of beef which is roasted in an oven. Roast beef is often served within sandwiches and sometimes is used to make hash.