The kilometre is a unit of length and is a multiple of the metre; an SI derived unit. It has the symbol km.
A kilometre is equal to 1000m, 1094 yards and 0.621 miles.
Kilometres is used to measure road distances in the US, though the UK is still using the imperial "sister" miles.
The parsec is a unit of length and is used in the Astronomical system of units. It is denoted by the symbol pc.
It is equal to 3.26156 ly (light years) which is approximately 31 trillion kilometres (19 trillion miles). It's used to measure huge distances to objects and planets outside of our Solar System.
It was first brought into use by Herbert Hall Turner (a British Astronomer) in 1913 and was invented to use in calculations of astronomical distances to make it easier in calculations using raw data.