Space Weather: Quiet vs. Active Times
The Sun seems pretty much the same from day to day. However, the Sun is actually changing all the time.
Dark spots show up on the face of the Sun from time to time. The number of these sunspots varies over the years. Records going back centuries show that sunspot counts vary dramatically and regularly in an 11-year cycle. When sunspots are few, solar storms are rare. When sunspots are plentiful, the Sun becomes unsettled and much more active. During peaks in solar activity huge explosions on the Sun (called solar flares) generate bursts of radiation and energetic particles. Active periods bring increased levels of ultraviolet radiation and X-ray emissions, more streamers and prominences above the Sun's surface, and an expansion of the solar atmosphere and heliosphere.
Solar activity affects the whole Solar System, including Earth. Higher radiation levels during active times are dangerous for satellites and astronauts. Solar storms generate beautiful auroral light shows high in our atmosphere - the Southern and Northern Lights! Solar activity can also interrupt communications, disrupt electrical power systems, and hasten corrosion of pipelines. Variations in the Sun probably influence our weather and climate, though we aren't exactly sure how yet.
The Sun also changes over longer timescales. The 11-year sunspot cycle has been interrupted at times, and some scientists believe there are longer-term cycles in solar activity. The Sun has gradually brightened (by roughly 30%) over its six billion year history. Our star was also much more active in its youth, displaying wilder variations in activity level as compared to modern times.