As you shop around for a gi you'll notice that some are referred to as traditional cut while others are tournament cut. Knowing the difference can be an important factor when determining which gi fits your needs the best.
Tournament vs Traditional is mostly a matter for karate uniforms as opposed to kendo or jujitsu. Therefore if you are shopping for a different art you needn't worry about it so much. But if you are shopping for karate the n you'll need to understand the cut of both uniforms. A tournament cut gi will have sleeves and pants cut shorter than traditional style. This was done for multiple reasons. First, the shortened limbs allow the judges to see the full breadth of the arms and legs during technique. This is believed to making judging easier.
Secondly, less gi material is believed to enhance mobility and even quickness during kata and kumite. It should be noted that there is no particular proof suggesting this is true. It could just be a matter of subtle psychology in believing there is less material to get in your way or slow you down.
Third, the shortened effects allow for easier application of sparring gear. Rather than having to slide the sleeves up to put on hand guards or worry about long pant legs you can easily slap on protective equipment.
Traditional vs Tournament Cut
It varies from brand to brand, but many tournament cut gi are also stitched to hug the body a bit closer. This is believed to show physique and form to the judges more effectively than a traditional cut.
Keep in mind when shopping that neither style gi is particularly superior. It's mostly about determining your needs and what feels best for you. There are no statistics to prove that wearing a tournament cut gi will get you more trophies, nor will wearing a traditional gi make you more historically relevant in your style. All of that comes down to the lineage and skill of the practitioner.
But looking good never hurt.