What is the difference between spot colors and global in illustrator? thanks
These refer to the ink process of how a color is printed.
If you're using CMYK (process colors), the colors on your page will be created by printing various amounts of either Cyan, Magenta, Yellow and/or Black on top of each other to create that color. But spot colors are their own ink color. So if you have a spot orange color, you're not printing CMYK values, you're printing just that orange ink.
It's guaranteed to be the exact correct orange in this case, but it's probably more expensive, and I would only recommend using it when the situation calls for it.
When does the situation call for spot colors?
When you're printing less than a total of 4 colors, and it really matters that those colors are exact, then use spot colors instead of any process colors. Instead of using CMYK, the printer will use those specific inks, and the cost won't go up. (Correct me if I'm wrong about that.)
If you're printing more than a total of 4 colors, your costs will start going up for every additional spot color used. For this reason, use spot colors strategically and conservatively. (Again, those with print shop experience can clarify or correct me on the specifics of pricing and ink availability.)
Global color swatches
Think of these the same way as paragraph styles and character styles for text. Global swatches are dynamic 'color styles' for objects.
For example, you draw three separate squares. You create a blue color swatch and make it global. You select all three squares, and click the blue color swatch. Now all the squares are blue. Deselect the squares. Now edit the blue color swatch and make it green. The squares become green.