A binary tree is Balanced if height of the tree is O(Log n) where n is number of nodes.
Balanced Binary Search trees are performance wise good as they provide O(log n) time for search, insert and delete.
A binary tree is balanced if, for every vertex, the number of vertices in its left and right subtrees differs by at most one.
Examples:
 AVL Tree
 RedBlack trees

Time Complexity 
Space Complexity 

Average 
Worst 
Worst 

Data Structure 
Access 
Search 
Insertion 
Deletion 
Access 
Search 
Insertion 
Deletion 

RedBlack Tree 
Θ(log(n))  Θ(log(n))  Θ(log(n))  Θ(log(n))  O(log(n))  O(log(n))  O(log(n))  O(log(n))  O(n) 
AVL Tree 
Θ(log(n))  Θ(log(n))  Θ(log(n))  Θ(log(n))  O(log(n))  O(log(n))  O(log(n))  O(log(n))  O(n) 