The difference between a fixed rate and an adjustable rate mortgage (ARM) is that, for fixed rates the interest rate is set when you take out the loan and will not change. With an adjustable rate mortgage, the interest rate may go up or down. Many ARMs will start at a lower interest rate than fixed rate mortgages. Because of this, often times, buyers that aren't planning to own the property for very long, will opt for an ARM.
More info at the Consumer Financial Protection Bureau site