One technique is called "scoring" or as some of us call "slashing". Something about being labeled a "bread slasher" seems to be a more appropriate description of some of us bread bakers. :) Scoring is a technique where a very sharp tool is used to cut through the "skin" of a loaf, usually after it has completed its final rise, but sometimes before. Direction of the cut can be straight down, at various depths or it can be done at an angle where the blade is actually under the skin of the bread, making a "flap". The final appearance of the baked loaf will vary depending on the pattern and angle of the scoring.
Various tools are used for making the cuts. There is a special scoring tool called a "lame" that uses a typical old fashioned double edge razor blade (looks dangerous to me). I've used a single edge utility razor blade. Others have used x-acto knives, scalpels, sharp paring knives....
What pattern of cut you use is limited only by your imagination. The bread loaf, whether in a loaf pan or on a baking sheet is a blank canvas ready for you to express yourself! Here is a simple 3 line design I scored on my loaf.