There are many abnormalities that can occur on the nails that indicate a number different things. Some of these conditions can mean a fungal nail, something more serious, or could mean nothing at all.
If you notice any ridges or lines on the nails this is may be normal if you frequently bite your nails, however if you notice them worsening this could also be a sign of a fungal nail. White powdery/brittle nails or yellow nails are also a sign of a nail fungus. The discoloration indicates air trapped under the nail. You can treat this by cutting the nails back, however if they seem like a severe case you may want to contact a podiatrist to see if you are eligible for a better treatment option.
If your nails appear red or black in color this is a sign of a hematoma, also known as blood beneath the nail. This typically is the result of trauma to the nail, such as hitting it against a hard surface, dropping something on it, etc. There is not much you can do in these cases other than wait for your nails to grow. If the discoloration remains and grows out on the new nail, you may have obtained a nail fungus, since trauma is a culprit of fungus. If you see black spotting on the nail without having any trauma then this should be addressed by a doctor immediately, as this may be a sign of melanoma.
These are just a few of many signs on the nails that could mean nothing at all or something more severe. When in doubt, it is always best to contact your doctor.