Economy Plus (E+) seats are regular coach seats with extra legroom. They're no wider and not physically in a different cabin or anything, but you get extra space for your legs, opening your laptop, etc. They advertise "up to 5 inches" extra - it varies by plane type. It's quite noticeable. You don't get nearly as many domestic upgrades on United as you do on American, but coach is much more comfortable because of E+.