What is backtracking and how does it differ from brute force?

Backtracking is a refined brute force that abandons partial solutions as soon as they're determined invalid, avoiding exploration of entire subtrees. Build solutions incrementally, at each step check constraints; if violated, backtrack (undo) and try next option. Unlike brute force that generates all possibilities then filters, backtracking prunes early. Used in N-Queens, Sudoku, permutations/combinations. Time can vary from O(n!) worst case to much better with good pruning.