"would" has some very precise grammatical uses which grammar books explain in detail. However, in most cases, it suggests that something is not certain, but possible or conditional on the happening of an other event. "will / shall" is used in all future tenses, generally when expressing future certainties.

would is used in some situations and conditional sentences where the outcome is unlikely. "I WOULD buy an airplane if I HAD enough money" most people dont have enough $ . "I WILL buy a car if I (ever) HAVE enough money." Notice also the verb "to have" is different If someone says: " I would buy I car if I had enough money" it means that they think it is unlikely they will have enough money.