Space weather is the study of changing environmental conditions in near-Earth space, or the space from the Sun's atmosphere to the Earth's atmosphere. It is a separate concept from weather within the Earth's atmosphere.
One of the events of most concern regarding space weather are coronal mass ejections (CMEs) from the Sun, which are considered separate events from solar flares - though the two appear to be related. CME's typically bring a shock wave of solar energetic particles to the Earth's atmosphere, causing a visual effect near the poles commonly called the 'aurora borealis'. Other effects include geomagnetic storms, radio blackouts, and electrical line overloads which can lead to cascading power outages, as well as satellite outages. Geomagnetic storms are categorized from G1 (minor) to G5 (extreme).