Differential settlement occurs when different parts of a building's foundation settle by different amounts, causing the building to sink unevenly. This can be caused by variations in soil strength or compaction issues. Uniform settlement across a building is expected over time but differential settlement can damage a building's structure. Signs may include cracks, sticking doors and windows, and leaning walls. Proper site inspection and using deep foundations like helical piers in expansive soils can help prevent differential settlement issues.