And & But

I've just started The New Penguin Russian Course and found something a little confusing:

(I don't have Russian installed so can't type it in Russian - so I'll type it phonetically)

a = and/but (indicating a slight contrast)
ee = and
no = but

When would you use "a" for 'and' or 'but' as opposed to "ee" and "no"?

http://masterrussian.net/mforum/viewtopic.php?t=8419

And now I know I've been at this forum for a while since I'm linking to old topics of mine...

I kind of figured it must have come up before :)

Thanks very much