Sunspots are localized regions of extremely intense magnetic fields. These magnetic fields intertwine, and the resulting magnetic energy can generate a sudden, violent release of energy called a solar flare or coronal mass ejections (CMEs).
Solar flares are associated with Coronal Mass Ejections (CMEs) which can ultimately lead to geomagnetic storms.

There is often confusion about the difference between solar flares and coronal mass ejections (CMEs).
The most obvious difference between a solar flare and a CME is the spatial scale on which they occur. Flares are local events as compared to CMEs which are much larger eruptions of the corona.
See Coronal Weather Report: CMEs and Flares | UC Berkeley

A solar flare is an explosion on the Sun that happens when energy stored in twisted magnetic fields (usually above sunspots) is suddenly released. Flares produce a burst of radiation across the electromagnetic spectrum, from radio waves to x-rays and gamma-rays.
Flares are characterized by their brightness in X-rays (X-Ray flux), the GEOS (Geostationary Earth Orbit Satellite) Class

Coronal Mass Ejections (CME) CMEs travel outward from the Sun typically at speeds of about 300 kilometers per second, but can be as slow as 100 kilometers per second or faster than 3000 kilometers per second. The fastest CMEs erupt from large sunspot active regions, powered by the strongest magnetic field concentrations on the Sun. These fast CMEs can reach Earth in as little as 14--17 hours.

A flare is called a coronal mass ejection (CME), an expanding bubble of charged particles that race outward. Flares release energy in many forms - electro-magnetic (Gamma rays and X-rays), energetic particles (protons and electrons), and mass flows. It arrives at earth 18-36 hrs. after leaving the sun. Light and other radiation travels from the Sun to Earth in about 8 minutes.

The Solar and Heliospheric Observatory (SOHO) coronagraphs captured this movie of a coronal mass ejection heading toward Earth on Oct. 22nd 2003.

Aurora, also called the Southern and Northern Lights, are created when the charged solar particles stream down Earth's magnetic field lines and excite oxygen and nitrogen atoms in the atmosphere. Normally the aurora are only visible from places near the poles, like Alaska. But when Earth's magnetic field is overwhelmed, the aurora can dip will into the United States and Europe.
See Northern Lights (Aurora Borealis)

Astronomers rank solar flares into five categories according to the extent to which they emit X-rays. Class X is the most powerful, being tens times the intensity of G-scale M-class flares.
An M5 is the strongest.

Kp-index:BR> A scale that ranges from 0 to 9, with 5 or higher indicating a geomagnetic storm. The Kp-index is derived from the maximum fluctuations of horizontal components observed on a magnetometer during a three-hour interval.
The May 2024 flare was a X8.7

G-scale:
he G-scale used by the U.S. National Oceanic and Atmospheric Administration, which rates the storm from G1 to G5 (i.e. G1, G2, G3, G4, G5 in order), where G1 is the weakest storm classification (corresponding to a Kp value of 5), and G5 is the strongest (corresponding to a Kp value of 9).
The May 2024 flare was a G-scale M5.

What is the M scale
The next level, ten times higher, is the "B" level (>= 10-7 W/m2); followed by "C" flares (10-6 W/m2), "M" flares (10-5 W/m2), and finally "X" flares (10-4 W/m2). Radio blackouts are classified using a five-level NOAA Space Weather Scale, directly related to the flare's max peak in soft X-rays reached or expected. Flares are characterized by their brightness in X-rays (X-Ray flux), the GEOS Class. The biggest flares are X-Class flares. M-Class flares have a tenth the energy and C-Class flares have a tenth of the X-ray flux seen in M-Class flares.

One on March 6, 1989 knocked out power to the Canadian province of Quebec.

See NOAA SPACE WEATHER SCALES

Source: Solar Cycle Progression | National Oceanic And Atmospheric Administration (NOAA) Space Weather Prediction Center

In 1997, an AT&T Telestar 401 satellite used to broadcast television shows from networks to local affiliates was knocked out during a solar storm. In May 1998 a solar blast disabled PanAmSat's Galaxy IV. Among the casualties: automated teller machines; gas station credit card handling services; 80 percent of all pagers in the United States; news wire service feeds; CNN's airport network; and some airline weather tracking services.

A space storm also heats the upper level of Earth's atmosphere, causing it to expand. That's no good for satellites that can get caught up in air that didn't used to be there. Communication disruptions can occur without actually damaging satellites. Even cell phone towers can be zapped, causing dropped calls.

The greatest solar storm on record (prior to the Nov. 2003 storm) occurred in 1859, shorting out telegraph wires and starting fires in the United States and Europe. Paal Brekke, SOHO deputy project scientist, told SPACE.com this week's storm, if it hooks up with Earth in just the right way, would be about one-third as strong as the 1859 tempest.

The magnetic orientation of the CME relative to the Earth's magnetic field affects the intensity of the flare.

Flares are characterized by their brightness in X-rays (X-Ray flux), the GEOS Class. The biggest flares are X-Class flares. They are major events that can trigger radio blackouts around the whole world and long-lasting radiation storms in the upper atmosphere.
M-Class flares have a tenth the energy. They generally cause brief radio blackouts that affect Earth's polar regions. Minor radiation storms sometimes follow an M-class flare.
C-Class flares have a tenth of the X-ray flux seen in M-Class flares. They have with few noticeable consequences here on Earth.

Strength is also determined by magnetic field strength of the solar wind measured in nT (nano-Teslas)

Date	Strength
1859	-1,760 nT. | shorting out telegraph wires and starting fires in the United States and Europe.
1967	-589 nT / X15 | Most extreme storm of the Space Age by several measures. Outed power grid of province of Quebec.[68] Caused interference to United States power grid. Blackout of polar surveillance radars during Cold War led U.S. military to scramble for nuclear war until solar origin confirmed
Jun. 1991	X12
Jun. 1991	X12
Oct. 1991	X6
Nov. 1997	X9
Aug. 1998	X5
July 2000	dubbed the Bastille Day event
April 2, 2001	> X17 Not pointed at Earth
Oct. 2003	X17 (may be the third strongest X-flare on record) - There were no significant effects.
Nov. 2003	X28 - Largest ever. Only part of the CME is directed towards Earth, so we will receive only a glancing blow, since the source region is pointing away from us on the right on the limb of the Sun as seen from Earth.
May 14, 2024	X8.7 - Northern Lights visible in central California and as far south a Georgia. The first M5 geomagnetic storm to reach Earth since October 2003

See List of solar storms Wikipedia See Astronomical Events
Oriented Scintillation Spectrometer Experiment (OSSE) Solar Flare Observations at nrl.navy.mil

See:
Astronomical Events
Coronal Weather Report: CMEs and Flares (Page 1)
http://news.bbc.co.uk/1/hi/sci/tech/411005.stm
http://www.space.com/scienceastronomy/solar_flare_031028.html
http://spaceweather.com/
http://www.sec.noaa.gov/
LASCO Coronal Mass Ejections Lists at nrl.navy.mil
http://www.sec.noaa.gov/SWN/
"Stormy Star", National Geographic, July 2004
Anatomy of Sun Storms & Solar Flares (Infographic) Large Image