meter - m kilogram - kg second - s ampere - A kelvin - K candela - CD mole - mol The above are Base SI units. Two supplementary units radian - rad steradian - Sr...
Units are related one to another. For example, since distance is measured in meters and time in seconds, velocity is measured in meters/second. That means you can apply the formula distance = speed x...
Sometimes you need a very large, or a very small, amount of a certain unit. One way to deal with that is to use prefixes like kilo, mega, milli, micro, and others.