A close relative of neon tetras, they have the same requirements (they prefer soft water, but can be adapted to live in neutral, they just can't breed in it) and almost look identical, except the red stripe along their abdomen goes throughout their body instead of ending halfway down the middle.
Like the neons they also have very stringent water requirements and can be difficult to raise (especially if you get a less hardy batch of them), as they do not tolerate change or fluctuations well (I do not advocate use of "pH down" type products to bring down pH; these usually stress them too much when the water pH bounces back up the next day due to carbonate hardness; if you are interested in lowering pH, do it more naturally by means of filtering through peat or adding driftwood).
However, between the two, I have found cardinals to be a bit hardier, and to grow faster. Some of this may just be batch-to-batch variation (perhaps how much they suffered during transport). Once you get them over the initial hump of adjusting to your tank environment and eating regularly and eagerly, they seem to do great for years. Often, people are warned against keeping neons or cardinals with larger fish who might seem them as potential food. I have kept mine with large angels 10X or more their size for more details, by buying the angels when they were a dime to a quarter size and getting them used to smaller fish; however, I have seen neons/cardinals get chased around to death by large angels which have not been raised with small fish and see them as prey.
These little fish are really striking in a large school of 10 or more (they need to be bought in schools of at least 4-6). They really do look like neon lights. But because of their beauty and now relatively inexpensive pricetag, a lot of beginners get them for newly setup tanks; I would not recommend them for a tank that is under 2-3 months old, especially if you want to buy the fish young (younger ones are usually more delicate, and prone to getting chased by other fish that take advantage of them).