Caulk and silicone sealants are materials used to fill gaps and cracks between surfaces, preventing water damage, air leaks, and pest infestations. Caulk is a flexible paste-like material for filling gaps around skirting, woodwork and interior windows, and it can be painted once dried. Silicone is a more durable and water proof sealant, ideal for bathroom and kitchen fixtures, and  exterior applications like sealing around windows, doors, and bathroom. It is not paintable.