How does the observable universe differ from the universe?
The observable universe is the part of the universe which we can see right now whereas the universe is the entire universe.
The observable universe is 45 billion light years in diameter and is actually 'shrinking'. Objects outside the observable universe are not seen because of this reason. The light that came from the objects outside the observable universe is trying to reach us but as the universe is expanding rapidly, the light fails to beat that expansion to reach us, therefore those objects are not visible. Right now, the furthest object visible is 13.7 billion light years away, but due to time and if you apply a little logic, the object is currently 45 billion light years from us.