Now that you've shaped your bread, you should know what direction the surface tension in your bread is pulling (imagine the gluten as the threads that hold your bread together, and they're stretched like elastic). In a bread loaf, or baguette, the tension runs width wise. In a boule, the tension pulls away from the center, like the spokes of a wheel.
To encourage the most rise, slash the bread parallel to the direction of the surface tension. If it is parallel, the bread will pull apart and away from itself where you have slashed and the slash won't close/stick back together. This is why a bread loaf is usually split/slashed lengthwise down the center, and a boule is slashed in a cross shape.
|Because this loaf was slashed lengthwise, it rose sideways in the oven.|
Sometimes you just want things to look pretty, right? Crisscrossed diagonal slashes on a loaf have the same effect as slashing down the middle, but gives the bread a different look. See how the bread still pulled apart down the center?
If you don't slash the bread, the dough will still try to rise, and will crack your bread wherever it can. Similarly, if you don't create large enough slashes / vents, it your bread will usually crack along the sides, and look like the top is going to pop off. Check out the photo below. The loaf split along the side because the slashes were inadequate.
And that's all you really need to know about slashing your bread before baking. We're getting to the good part now. It's almost time to eat!
Start from the beginning: Sourdough Workshop Index.