A meter is the SI unit of distance. The meter is defined as the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. The meter is defined as a measure of time because time can be measured far more accurately than length (at the time of the definition, time could be measured up to 1 part in 1013 whereas length could only be measured up to 4 parts in 109).
Metric prefixes make longer units like kilometer, km (1000 m) and megameter, Mm (106 m) and smaller units as well like millimeter, mm (0.001 m) and the μm (10-6 m). The μm is called both the micrometer and the micron. The word micrometer is ambiguous since it's also a device that measures very precise lengths, hence the two terms.
Read more about meters Dr. Rowlett's unit dictionary.