Light-second

The distance that light will travel in a vacuum in one second.  Aproximately 166,000 miles, or 3 x 10⁸ metres.

Light-minute

The distance that light can travel in a vacuum in one minute

The distance that light can travel in a vacuum in one hour

Light-week

The distance that light can travel in a vacuum in one week

Light-year

The distance that light can travel in a vacuum in one year

Astronomical unit (AU)

The distance from the Sun to the Earth - approximately 8 light-minutes, see "The sun & Planets"

Parsec

The distance of astronomical objects is determined by measuring their position in the sky when the earth is at opposite points in its orbit.

Draw a triangle.  One point is the earth on one edge of its orbit, the second is the sun.  This line is thus 1 Astronomical Unit long.  The third point is chosen so that the angle between the lines from it to the sun and the earth (its parallax) is one second of arc.  That object is one parsec away. 

Fig 1 - An object is one parsec away
when angle x is one second of arc.

A parsec is equal to approximately:

206,265 Astronomical units,
3.26 Light-years,
1.917 x 10¹³ miles, or
3.068 x 10¹³ kilometres.

Some web-sites I visited researching this article (including some "encyclopaedia" sites that should have known better) suggest that the parsec is the distance when the angle measured between opposite points on the earth's orbit is one second.  Not only are they wrong, but their maths don't add up.