Question

What is one different between fixed- rate mortgages and variable -rate mortgages

Answer 1

Right in the names, it's clear :)
Fixed-Rate = the interest will not go up or down, it will stay the same, indefinitely, or until the mortgage is paid off.
Variable-rate (It's adjustable) = the interest may or may not get higher or lower. No definite interest rate

Answer 2

minor correction to the above fixed rate not iindefinite but for the life of that loan.