Every organisation is composed of certain parts. These parts then have their various functions and are interdependent on each other for a smooth functioning of the organisation. An organisation's structure is a framework that allots a particular space for a particular department or an individual and shows its relationship to the other. An organisation's structure may be of many types, the most common of these being the hierarchical and the flat organisational structure.
A horizontal organisational structure is what we call the traditional structure or at times, the bureaucratic structure where there are one or more levels between the most junior and the senior most employees. This helps in proper distribution of work but can be harmful in terms of efficiency and decision making.
A flat organisation is much more relaxed and so-called modern in approach where everyone directly reports to a single boss. This could provide greater speed in the decision making process but then the boss ends up taking care of a lot of things thus making delegation difficult.