In the world of computers, the central processing unit (CPU) is–well–central. Your first computer course probably explained it like the brain of the computer. However, sometimes you can overload that brain and CPU designers are always trying to improve both speed and throughput using a variety of techniques. One of those methods is DMA or direct memory access.
As the name implies, DMA is the ability for an I/O device to transfer data directly to or from memory. In some cases, it might actually transfer data to another device, but not all DMA systems support that. Sounds simple, but the devil is in the details. There’s a lot of information in this introduction to DMA by [Andrei Chichak]. It covers different types of DMA and the tradeoffs involved in each one.