It's no different from traditional United Economy service. You simply receive improved legroom. The only way to be seated in Economy Plus, though, is:
1 to have Premier, Premier Executive, 1K, UGS, or Star Gold status.
2 to be traveling with/have your ticket booked by an elite-status customer, as described above.
3 to pay a full-fare Economy Class ticket.
4 to ask for it specifically at check-in (if you check-in early enough, this often works).
It is currently available onboard all United mainline equipment, save for the 767-200s and selected International 3-class 777-200 equipment.
Enjoy the friendly skies of United!
[Edited 2004-01-10 08:37:23]