A balanced binary tree is the binary tree where the depth of the two subtrees of every node never differ by more than 1.
A complete binary tree is a binary tree whose all levels except the last level are completely filled and all the leaves in the last level are all to the left side.
Below is a balanced binary tree but not a complete binary tree. Every complete binary tree is balanced but not the other way around.
1
1 1
1 1 1
1
As implies, in a complete tree, always the level difference will be no more than 1 so it is always balanced.