In Photoshop, a lot depends on the tool you a re using to draw your "line". In general, anti-aliasing options are on the top bar, if the tool you are using allows anti-aliasing.
However, there are a lot of other approaches. It's always possible to draw a rather jagged line -- even though its edges may be antialiased, you still don't want am erratic line with smooth edges.
-- For a straight line from any one point to any other point --
Click on one end point, hold down SHIFT, and click on the other end point. The resulting line will be straight, but if it's on an angle, it will NOT be anti-aliased. You'll see the jaggies along the edges of your tilted line, but the line itslef will be straight.
-- For a perfectly straight horizontal or vertical line --
Hold down SHIFT while you drag the tool. It will be constrained perfectly vertical (or horizontal) no matter how your mouse jitters.
This becomes very intuitive very quickly when you start playing with it.
Found on the pencil tool tab, and works similarly to the Pen tool. But the edges of the line WILL be smoothly anti-aliased -- and even a bit blurry, depending on which brush you select.
This is the best way to draw smooth curves. It creates a vector path which you can do a lot with after it's created - tracing the path with a color, turning it into a selection, and so on.
It's best to learn about the pen tool and paths through the Photoshop Help menu or a tutorial. But for smooth curved lines, this is the best approach.
You click with the pen tool to put down one anchor point for the curve, and then click again to fix a second point. When you click the second point, you get a set of handlebars that you can twist and turn, creating different smooth curves between the two anchor points. Now you can create a third anchor point, etc.
It does take some practice to become fluent with the Photoshop Pen Tool. It works essentially the same as the tool in Illustrator, so if you're familiar with that program, you've got a head start.
[edited by: tedster at 5:07 am (utc) on June 6, 2003]