When I'm cycling I fiddle with the gears until I feel like I'm making better progress but I rarely know what exactly I'm doing.

I have 3 front gears on the left handle bar and 8 back gears on the right handle bar.

Going down a steep hill I am always on 3 and 8 and going up a very steep hill I might have the left at 1 and the right 3, 2 or 1

What is the difference between the having left at 3 and right at 1 and having the left at 1 and the right at 8?

Also are there any strategies you cyclists use as well? Like when you see a hill coming etc.?