To be honest, many of them are self-explanatory, but briefly:
a:link etc. = standard hyperlinks
.nav etc. = navigation tree hyperlinks
table & td = tables and table cells (lots of stuff in tables!)
input = post subject input, attachments field, some buttons and other input fields
textarea = main post field, edit template field
select = drop-down choices
.windowbg = posts and stuff
.windowbg2 = posts, news, category descriptions etc.
.windowbg3 = some horizontal rules (eg post dividers)
hr = horizontal rules
.titlebg = various titles
.text1 = not much, if I remember right
.catbg = category titles
.bordercolor = borders for tables, created by other tables
.quote = quoted post boxes
.code = code boxes
.help = help cursor
.meaction =
* Peter Duggan has just demonstrated this!
.editor = width of some input fields
What I'd suggest doing, however, if you want to try things out, is to take a look at Andrea's
YaBB board color helper, which should show you more graphically than mere words!
<edit>Corrected spelling of 'bordercolor'.</edit>