I have been a Windows user for probably all my life, but started using Mac a few years ago. While I never required to use an external mouse with my Mac earlier on, I did find the need for one when I started using the Mac for longer periods.
On Windows, a mouse with a scroller will scroll the page in the direction the mouse wheel is scrolled, for example; the page will scroll down when you scroll the mouse wheel downwards and vice versa. However, Apple uses something called as natural scrolling direction and scrolls the page in the opposite direction.
If you are someone who is annoyed because of this strange scrolling behavior on a Mac, you can change the settings to make the page scroll in the direction you scroll the mouse wheel in.
To make that change, head over to System Preferences -> Mice and then uncheck the box next to “Move content in the direction of finger movement when scrolling or navigating” (In older Mac OS X versions, this would be the checkbox next to “Natural” scrolling”.
Once you uncheck the box, the page will scroll in the direction of the mouse wheel and not the opposite direction.