A light year is a measure of distance, or the distance a particle of light will travel through a pure vacuum within one Earth-year.
This distance is approximately 5,874,589,152,000 [5.87 trillion] miles, or 9,454,254,955,000 [9.46 trillion] kilometers.
In a vacuum, light travels at 186,000 miles every second so in a minute it will travel 60 times this amount (as there are 60 seconds in a minute). In 1 hour (60 mins) it will travel 60 times this amount. In 1 day (24 hrs) it will travel 24 times this amount. In 1 year (365 days) it will travel 365 times this amount. This amount is equivalent to around six million, million miles.
The nearest star (Proxima Centauri) is 4 1/4 light years away or approx. 25 million, million miles. The galaxy is 100,000 light years across The known universe is over 13 billion light years across. If a star is 10,000 light years away, we are seeing it today as it was 10,000 years ago – as the light that started out then is only just reaching us. Similarly, if there we aliens looking at us through a powerful telescope on a planet that was just over 7 light years away, then they would just today, in 2008, be seeing 9/11 happening.
The measurement of a light year is useful in finding distances to very distant objects, and as we’re assuming that there is a pure vacuum between us and whatever you’re looking at, we can then determine how long ago, in Earth years, that light originated from whatever we’re looking at. It is entirely possible that some of the stars we can see from Earth are simply no longer there!