A spit is a bar of sand or other sediment that is still connected at one end to the mainland.
The sand is deposited by waves, currents, and tides. Barrier spits are actually bars that cut off a small part of water inland; the body of water becomes a lagoon.
Double spits look like barrier spits except that they do not completely meet as they come out from shore. They don’t actually cut off water to form a lagoon.
Tombolos are spits that connect the mainland to an island. Cape Canaveral, Florida, the site of many rocket launches, is actually a triangular spit of sediment called a cuspate foreland.