請問這兩個字的用法有什麼不同？

Usage:
Both “will” and “going to” are referring to the future.

A. Going to
1) planned actions in the future
E.g. We are going to Japan at coming Christmas.

2) You are certain that something is going to happen in the future.
It is already 9 a .m., I am going to be late for school.
Format
Going to + infinitive

B. Will
1) Future actions happen without the speaker's intention

The sun will shine tomorrow.

2) Predictions, assumptions

I think John will wait for us.

3) Spontaneous actions (Actions happen at the same time)

Stop crossing the road, a car is coming.

Format:

will + infinitive

will=be going to (going to 前記得加be動詞)
都是未來式的用法,文法上用法可完全互換,沒什麼很大差別

Will 表意志
Going to 表行動