Probiotic supplements can be found in the shape of capsules but also as shakes perhaps even foods and the best thing about probiotics is you can get them naturally from foods like pickles and other fermented vegetables, kefir, yogurt and more. Probiotics have quite a long history which dates as far as 1905 when they were first discovered by Russian scientist Élie Metchnikoff, also the Nobel prize recipient for discovering white blood cells.
Despite having such a long history, they were not as popular or as studied as they are nowadays, we even have specialty stores which sell only probiotic products. What caused this increase in success and popularity is the evidence from clinical trials and scientific studies which proved that probiotic bacteria can be used for many purposes and have a big potential in health management and preventive medicine especially.

In 1905 scientist Élie Metchnikoff observed that the villagers in the countryside of Bulgaria lived healthy and long lives, longer than what was considered average life-span and he associated this with their regular consumption of yogurt, a product that contains natural strains and CFUs of probiotic bacteria in it and he suggested that these strains of healthy bacteria could prevent or help in treating health problems and improve or promote a better digestion, studies today show that he was very right, but they could possibly do more than that, however we lack information, research, and evidence on them but as it stands now with their popularity and success, they will probably be more researched in the future and we will learn more about them.
Some scientists suggest that we could use probiotic bacteria as antioxidants, to reduce oxidative damage or oxidative stress in the cells in the human body.

Others even think that they could have a role in preventing cancer cells, but this is one of those claims that needs more evidence and more research, right now most people use probiotics for improving digestion, treating digestive or gastrointestinal infections and also for their immune support.
For those of you interested, visit Nucific and check out their products, I have used one myself and I highly recommend it, it's called Bio X4 and it has 5 different strains with 4 billion CFUs/capsule.