How easily you can figure out the interface on your own.2. Efficiency
How quickly you can accomplish your tasks.3. Memorability
Having used the software at least once, how easily can you recall how to use it the next time you use it?4. Error rate
When using the software, how often users tend to make mistakes.5. Satisfaction
If the software is pleasing to use.However, that last item is treading on different territory: User eXperience (UX). And UX is different from usability.
If usability is about real people using the software to do real tasks in a reasonable amount of time, User eXperience is more about the user's emotional response when using the software. Or their emotional attachment to the software. UX is more closely aligned to the user's impression of the software beyond usability, beyond using the software to complete tasks.
Usually, usability and UX go together. A program with good usability tends to have positive UX. A program with poor usability tends to have negative UX. But it is possible for them to differ. You can have a program that's difficult to use, but the user is happy using it. And you can have a program that's very simple to use, but the user doesn't really like using it.
Let me give you an example: a simple music player. It's so simple that it doesn't have a menu. There's an "Add songs to playlist" button that seems obvious enough. The play and stop buttons are obvious (a button with the word "Play" to play music, and a button next to it labelled "Stop" to stop playing music.). To change the volume, there's a simple slider labeled "Volume" that has "quiet" and "loud" on each end of the slider.
It's easy to use. The music player is obvious and well-labeled. You can imagine it scores well with Learnability, Efficiency, Memorability, and Error rate.
But it's bare. There's no decoration to it. The program uses a font where the letters are blocky, small-caps, and spaced very close together. It uses the same font in the music list.
And the colors. Everything is white-on-black. The "Play" button is a sort of sickly green, and the "Stop" button is a sort of reddish-brown. The "Add songs to playlist" button is a weird purple. The box that shows the music play list is an eerie green.
The program works well, but you just don't like using it. Every time you use the music player, your stomach turns. The colors are depressing. As soon as you load your play list and start playing, you cover the window with another window so you don't have to look at the program. After you use it a few times, you switch to another program. Even if the other program isn't as easy to use, at least you'll like using the other music player.
So that's one example of a program that would have good usability but negative UX.