In this review, an attempt has been made to describe the large variety of geomagnetic variations, both regular and irregular. After a brief description of the Earth and its environment, different types of quiet-day variations are described and present ideas regarding their possible mechanisms are discussed. In general, periodicities exceeding several tens of years can be attributed to changes in the interior of the Earth while periodicities of 22 years or less seem to be related to phenomena connected with the Sun, through the interaction of solar wind with the Earth's magnetosphere. The morphology of irregular storm-time variations and its relationship with interplanetary plasma parameters is discussed with particular reference to the orientations of interplanetary magnetic field, particularly the southward B z component which seems to play a crucial role. Various storm-time phenomena occurring in the polar, auroral, mid-latitude and equatorial regions and their interconnections are described. Theoretical models offering explanations of many of these phenomena are discussed, the unsolved problems are outlined, and the direction of the present effort in solving these is indicated.