Most vehicles have 1-5 lock pins, depending on age and sophistication. Commercial trucks will have no lock pins, hence are the quickest and easiest to steal. Cheap cars made by the mid-1930s will have 1 lock pin. The most expensive luxury sedans and limousines will have 5 lock pins.
Lock picking is not strictly limited to cars. Some buildings have pickable doors too, in the Office of Price Administration, the back door has 4 lock pins.
It is said that the number of lock pins on a car gives the player an idea of how superior or basic the quality of the car is, much like how 1-5 stars rate a hotel's quality, service and amenities. Therefore, cars can be informally rated by a 1-5-lock pin rating system.
Lock Picks are first seen in Home Sweet Home where Vito Scaletta buys some so he can steal a Jefferson Provincial. After they are bought you have them forever. They are still in Giuseppe's Shop even though you cannot buy them again.
Lock picks can be used to pick the locks of vehicles and sometimes even doors.
You can buy them again for $10.00 if you go to Giuseppe's shop in the beginning of Chapter 7 In Loving Memory of Francesco Potenza before seeing Joe but you'll need to rob a store before doing so.