Seals are generally moulded or machined product, often flat and round such as an O-ring. Gaskets are cut into different shapes so that they fit the design and bolt spacing of a component.
Gaskets are used to seal two components or flanges that have a flat surface. Seals are used to describe parts that are used between engine parts, pumps and shafts that rotate. Gaskets are used as static seals.
When a Seal is used between engine parts, pumps and shafts that rotate, it is described as a dynamic seal. Seals are required to keep leaks from occurring within a moulded or machined product.
When a gasket breaks down or a machine is serviced the gaskets need replacing. A seal within a bearing needs complete replacement of both the seal and bearing if there is a seal breakdown.