GTA 5 and GTA Online boast a huge selection of vehicles; knowing how to save them is essential.
In GTA 5's story mode, use the garages attached to each protagonist's safehouse to store your cars.
To keep your GTA Online vehicles, you'll have to buy properties with garages, either apartments or standalone garages.
Be aware that certain stolen cars in GTA Online cannot be stored; purchasing them is the only way to keep them.