When the engine is cold, fuel vaporizes less readily and tends to condense on the walls of the intake manifold, starving the cylinders of fuel and making the engine difficult to start; thus, a richer mixture (more fuel to air) is required to start and run the engine until it warms up.
On most Carburetors a choke mechanism is employed to restrict the passage of air thru the venturi, increasing the fuel mixture. Other carburetors, notably the Gurtners found on Motobecane and Peugeot moped engines employed an enrichment circuit, which increases the amount of fuel available instead. Bing carburetors employ a standard choke slide as well as a third enrichment method, a tickler. This is a pushrod mechanism manually opens the float valve and allows more gas to enter the float bowl. An excess of fuel in the float bowl can cause a richer mixture.
The enrichment circuit is commonly referred to as a "choke" regardless of which mechanism it employs to enrich the mixture. Choke mechanisms are either hand actuated or cable actuated, depending on manufacturer and configuration.