Are increase and decrease arrows redundant on scrollbars?

The arrows on scrollbars are a functional element. They are therefore not redundant as nothing else behaves in the same way.

1. Which are the two types of scroll bars and what is the difference between them?
2. How does a scroll bar work?

Which are the two types of scroll bars and what is the difference between them?

Based on their orientation, there are two types of scroll bars: horizontal and vertical. The horizontal scroll bar allows the user to navigate a document left and right. The vertical scroll bar allows navigating up and down.

How does a scroll bar work?

A scroll bar's orientation determines the direction in which scrolling occurs when the user operates the scroll bar. A horizontal scroll bar enables the user to scroll the content of a window to the left or right. A vertical scroll bar enables the user to scroll the content up or down.