If things of the same type vary, they are different from each other, and if you vary them, you cause them to be different from each other.

Salary scales vary between states/from state to state/according to state/with each state. 工资高低各州／州与州／不同的州／每个州都不同。

The samples varied in quality but were generally acceptable. 这些样品质量有差异，但是大体上可以接受。

We have tried several different approaches, with varying degrees of success. 我们尝试过不同的方法，成功率也不同。

to change or cause something to change in amount or level, especially from one occasion to another

My taste in classical music varies greatly/widely, but I usually prefer Mozart or Brahms. 我对各种古典音乐的喜好程度很不相同，但我通常比较喜欢莫扎特或勃拉姆斯的音乐。

Some people give a regular monthly donation while others vary the amount they give. 有些人每月捐赠固定数目的钱款，而有些人每月捐赠的数额多少不一。