So much is written about the usability of web sites that people forget that usability isn't just about web sites. Usability engineering applies to anything and everything that people use. It's a science of understanding humans and how they interact with things, a way of measuring how effective of a tool we've given the user, and a way of designing products and engineering improvements to benefit the users.
Take my tent, for example. I've been camping since I was a tyke and setting up a tent used to be a pretty arduous job. Something a kid could handle, but it quite a bit of time and some assistance from the parent-units. Then someone thought of those magic snap-together, bendable poles with elastic ropes in the middle to hold them together--that made setting up your tent much, much easier. Tents became almost an instant popup, and once you got your snapping, bendy poles in place, you could pick up your tent and move it anywhere you want it.
Problem was, you had to string those poles through these long, fabric tunnels on top of the tent and most of the time they would come apart. Kinda frustrating. There are still a lot of tents out there like that--my son got one for Christmas last year. He can set it up, but he needs a little help.
My parents gave me a tent, though, that eliminated that problem entirely. You snap together the frame, then hook the tent onto it. The rainfly drapes over the frame, hooks to the front and the back of the tent, and you snap in the corners like you would a seatbelt. It's amazingly simple--one of the most intuitive user interfaces I've ever met! The tent itself is a pretty plain vanilla, one room/4-person tent, but I love it. It's got a little hanging pocket right by my pillow to put little things like my glasses and my flashlight in at night, and it's so incredibly simple to set up! Who wants to spend your vacation time setting up tents? Tent users want to get that part over with as quickly as possible and move onto the FUN.
The people who designed that tent took usability into account. They thought of a better way to meet the user's goals of spending more of her limited vacation time having fun (and less time setting up camp)--and believe me, the users really appreciate that.
My point here is not to think of usability engineers as "web designers". Usability is important in web design, but it is much farther-reaching than that, and it's a lot more involved. Usability is a blend of human psychology, scientific research, and systems engineering. It's often called human-computer interaction (HCI), or human factors engineering.