Input lag or Game Delay

Hello everyone, since I started gaming I have been faced some problems such as latency, fps, etc..
its all common problems which can be solved with help of a friend or online but now I came across to this thing called 'Input lag' which is affecting my game-play big time, it might be I'm growing old or it has something to do with the computer itself.

Basically, I encountered this problem in different CPU's in different places such as 'lan-cafes', my friends computer etc.. it's different for all of them, I feel like My CPU is not instant like it should be when I play, which I feel hesitated to do somethings which needs immediate actions, such as flicking/ spray reaction or strafing (fast).

I asked this question for so many people and most of them has no clue that it is exist, I asked friends of mine and suggested some programs that I should download a program in order to check files or running programs in my computer.

To my knowledge just like most obvious answers were pointed to Screen Monitor, but the question : Are there more things other than that ?

I'm only here to ask if anyone can provide information related to this mysterious problem.