Short answers, pulled from the story.
The Severan dynasty was founded by Septimius Severus and his wife Julia Domna. Septimius, born on the 9th of April, 145, in Leptis Magna in the Roman province of Africa Proconsularis, seized power in 193 after emerging victorious from the civil war known as the Year of the Five Emperors.
Caracalla became sole emperor by murdering his younger brother Geta in 211, shortly after their father Septimius Severus died. Geta was killed in their mother Julia Domna's apartments, and Caracalla then purged Geta's supporters while buying the loyalty of his legionaries with pay raises.
Elagabalus, who became emperor at age 14 in 218, replaced Jupiter with the Syrian sun god Elagabal as Rome's supreme deity and married a Vestal Virgin named Aquilia Severa, both serious violations of Roman religious tradition. Ancient sources also accused him of effeminacy and licentiousness, though modern historians question how much of that record is reliable.
Julia Maesa, sister of Julia Domna and grandmother of both Elagabalus and Severus Alexander, was instrumental in placing and preserving Severan power. She used her personal wealth to secure army loyalty for Elagabalus in 218, then later forced Elagabalus to adopt Severus Alexander as heir when his behavior threatened the family's hold on power.
The Severan dynasty ended in early 235 when Severus Alexander and his mother Julia Mamaea were murdered by their own troops while Alexander was wintering in Germany to wage war in Upper Germania. His successor was Maximinus Thrax, whose accession began the Crisis of the Third Century.
The Severan dynasty ruled the Roman Empire from 193 to 235, a span of roughly 42 years. The dynasty was briefly interrupted in 217-218 by the reigns of Macrinus and his son Diadumenian before Julia Domna's relatives reclaimed imperial power.