This article reviews spintronics based memories, in particular, magnetic random access memory (MRAM) in a systematic manner. Debuted as a humble 4 Mb product by FreeScale in 2006, the MRAM has grown to a 256 Mb product of Everspin in 2016. During this period, MRAM has overcome several hurdles and have reached a stage, where the potential for MRAM is very promising. One of the main hurdles that the MRAM overcome between 2006 and 2016 is the way the information is written. The 4 Mb MRAM used a magnetic field based switching technology that would be almost impossible to scale below 100 nm. The 256 Mb MRAM, on the other hand uses a different writing mechanism based on Spin Transfer Torque (STT), which is scalable to very low dimensions. In addition to the difference in the writing mechanism, there has also been a major shift in the storage material. Whereas the 4 Mb MRAM used materials with in-plane magnetic anisotropy, the 256 Mb MRAM uses materials with a perpendicular magnetic anisotropy (PMA). MRAM based on PMA is also scalable to much higher densities. The paper starts with a brief history of memory technologies, followed by a brief description of the working principles of MRAM for novice. Reading information from MRAM, the technologies, materials and the physics behind reading of bits in MRAM are described in detail. As a next step, the physics and technologies involved in writing information are described. The magnetic field based writing and its limitations are described first, followed by an explanation of STT mechanism. The materials and physics behind storage of information is described next. MRAMs with in-plane magnetization, their layered material structure and the disadvantages are described first, followed by the advantages of MRAMs with perpendicular magnetization, their advantages etc. The technologies to improve writability and potential challenges and reliability issues are discussed next. Some of the future technologies that might help the industry to move beyond the conventional MRAM technology are discussed at the end of the paper, followed by a summary and an outlook.