When visitor is not login and add some product to his/her cart then we store that content temporary for them and if visitor leave site then that content is removed.

When visitor is already register with us then if he/she add some products in cart then that products remain in cart till visitor finish his checkout or manually remove products.

If visitor is already registered with us and add some products in his/her cart, then if visitor login then we automatically merge his existing cart products with already added products in past.