Originally Posted by: earthman buckbut isn't it true that the notes in a scale can be played along with other notes?[/QUOTE]

Yes it's true, especially in jazz to play notes that aren't in key and don't fit in the scale, but the important thing is this: Tension and Release. When you play a note that 'doesn't fit', it will sound tense and unresolved, but if you resolve it to a note that does fit, it will make sense.

[QUOTE=earthman buck] If so, what do the notes on a scale mean? And also, how do you know which scale(s) to use when improvising?

When you play a chord, there will pretty much always be a scale that will go with it, sometimes more than one scale.
Finding which scale to play over a chord becomes easier if you know what key the song is in. For example, if I were to play a Gminor7, there are several potential scales you could play over that, including the G Aeolian(pure minor), G Dorian and G Phrygian. Now, those scales all relate to a different parent scale or key. The G Aeolian is the 6th mode of the Bb major scale, the G Dorian 2nd mode of the Fmajor scale etc.

I find it hard to explain theory in text sorry. Hope I helped, even a little bit.