Attire
The names of these art forms tell pretty much all you have to know about the attire. Gi and No-Gi BJJ are the 2 forms.
The one in which a uniform or Kimono or Gi is a must is called the GI Jiu Jitsu. The GI has two parts a jacket and trousers which are made of cotton or ripstop cotton. The jacket is open, it does not have any buttons, hooks, loops or zippers while the trousers have a drawstring closure. The belt, which is also made of cotton, is tied around the waist which supports the jacket and pants. Actually, it is used to show your current skill level. You can choose to wear a rash guard under the jacket or go bare-chested. Females have the option to wear a bra with a rash guard, or a sports bra.
While the one in which you don’t wear a GI, instead wear some rash guards and spats is called No-Gi Jiu Jitsu. No-Gi Jiu Jitsu both male and female wear rash guard with either grappling shorts, compression shorts, or spat pants with shorts or just the spats. Neither the top nor bottom apparel have any pockets, zippers, buttons, hooks or loops.
Grips
In Gi Jiu Jitsu you can grab the clothing of the opponent while in No-Gi grabbing of the clothes is not allowed. The No-Gi apparel material is not made for grabbing or to be used for advantage. But this does not mean that you cannot form a grip in No-Gi. So, instead of grabbing the clothing you would grab the wrist, back of the neck, back of the ankle, and bend of the elbow. While in the other form you can take help by grabbing the clothing form a grip.
Movement and control
No-Gi practitioners are quicker and more explosive than Gi Jiu Jitsu practitioners. The fact that you cannot grab the clothing makes it easier to slip out of positions. So, there is a lot more movement and a less secure grip. But that makes the No-Gi practitioners quicker, the grips more powerful, and movements fast.
Techniques
Lapel chokes, collar grips, pants grips, sleeve grips, leg lasso, spider guard, worm guard, and many more techniques are not possible without the GI. But on the other hand, without the GI takedowns and neck chokes are easier because there is no fabric around to protect it. Other techniques like armbars, triangles, and naked rear choke are easier to escape from too.
Tournaments
Both Gi and No-Gi tournaments are points-based. But Gi Jiu jitsu is more popular having more competitors. The IBJJF is for GI fought in the Gi and ADCC is fought in NO-Gi. Both of these are the most prestigious event in their respective world.
Of course, there is a change in rules. For instance, in IBJJF heel hooks aren’t allow for Gi and allowed in ADCC. To become a better fighter, I recommend you should train both. You have a choice on what to train first.