What is a GPU and do you need one in Deep Learning?

Any data scientist or machine learning enthusiast would have heard, at least once in their life, that Deep Learning requires a lot of hardware. Some train simple deep learning models for days on their laptops (typically without GPUs) which leads to an impression that Deep Learning requires big systems to run execute. This has created a myth surrounding deep learning which creates a roadblock for beginners. Every book that I've referred to in the past few years has the author always mentioning the following: Deep learning requires a lot of computational power to run on. But I don't have datacentres at my command and when I built my first deep learning model on a sizable laptop, I knew that the consensus was either wrong or portrayed with some truth.