he's a hacker, so i think it's data from a company's network. thinking about it, it could also be a virus. i'm pretty certain it's explained earlier in the film (or later, when the agents are at the office), but i haven't seen the film in a while so i can't be sure
What Neo sells at the door of his apartment is a literary device.
It doesn't matter what it is - it's just a reason for the white rabbit to turn up. It can be whatever you want. I think it's a mini-dvd of Matrix Reloaded.