Deep foundations are structural elements that transfer building loads to deeper, more stable soil or rock layers. They are used when surface soils cannot support the weight of a structure, such as in tall buildings or bridges. Common types of deep foundations include piles and caissons, which are driven or drilled into the ground. These foundations help prevent issues like settling or shifting, ensuring the stability and safety of the structure. Engineers assess soil conditions and load requirements to determine the appropriate type of deep foundation for a specific project, making them essential in construction in challenging environments.