The directed energy deposition (DED) process using laser beams is an upcoming technology to produce additively manufactured structures. An overview and categorization are given including a brief history of the process and the previous processing technologies that enabled DED. The process principles with possible variations are introduced. The most relevant principles of laser light generation, delivery and focussing are discussed as introduction to the tool laser, followed by a description how photons interact with different surfaces relevant for the DED process and how a melt pool is formed for powder incorporation. Related monitoring possibilities in research and industrial environments are introduced. The most relevant features of the DED process of material addition are described in detail supported by several high-speed imaging results, describing the powder interaction on solid and liquid surfaces and the powder incorporation process of different metal materials. In addition, surface effects are introduced, such as oxidation of surfaces and the impact on powder particle incorporation. Addressing the user group of DED, an overview of the most relevant impacts of process parameters for different materials is given. Typical process imperfections and challenges are discussed, such as pore and spatter formation supported by high-speed images.