Answer
An earth year is a unit of time.
A light year is a measure of distance.
Explanation:
A light year is the distance light will travel in a vacuum over the period of one (earth) year.
Since light travels at about 186,300 miles per second, with 86,400 seconds per day and about 365 days per year, that works out at about:
$$ 186300 \times 86400 \times 365 \approx 5, 875 , 000, 000, 000 $$ miles