Why does my terminal show a string like this?

When I open my terminal it shows this:

wpa-2-602:~ david$

I just curious what the wpa-2-602 means? "david" is my user name, and sometimes it was something else. What makes it change?

That's based on the PS1 variable set in Bash's initialization. Yours appears to be the default, which is your computer's name, followed by a colon, then the current directory (in your example, that'd be the home directory for your computer [~]), then your username (david).

Here is an article about customizing your PS1, and there are plenty of questions both here and over at Super User about changing and fixing it.