Freediving is a form of underwater diving where you do not use any breathing appartures like in scuba diving. Instead, you take a deep breath and hold it until resurfacing. Even though in principle everybody is able to just hold their breath and dive there are several organisations (like AIDA or SSI) who developed a training program and offer freediving certifications. From my point of view it is definitly worth doing one of these certifications. Freediving is a sport with certain risks you need to understand to practice it safely. Besides, a bit of theory and proper technique will also help you to progress a lot faster in this sport.
There are several freediving disciplines ranging from just holding your breath over covering horizontal distance to going as deep as possible. Especially the depth disciplines are slightly different in certain ways. Some of these are with fins and others are without. At some you are allowed to touch the line and at others you are not. So there is different tehnique and also equipment involved depending on the discipline. In general, the equipment used in freediving is slightly different than what is used for scuba diving. The fins are usually longer and the wetsuits are in general more flexible. Furthermore, the masks have a smaller airvolume for easier equalisation.
Freediving is getting more and more popular and there are several spots in the world which have the perfect conditions to practice this sport. So if you are not already a freediver I can only strongly recommend trying out this amazing sport.