What is the difference between cohesive and cohesionless soils?

Cohesive soils (clays, silts) have particle attraction due to surface forces and can maintain shape when excavated. They exhibit plasticity, time-dependent behavior (consolidation, creep), and undrained strength. Cohesionless soils (sands, gravels) have no inter-particle attraction and derive strength from friction at contacts. They drain instantly, don't exhibit plasticity, and strength depends on density and confining pressure. Foundation design approaches differ significantly for each type.