Short answers, pulled from the story.
The Empire of Thessalonica was a short-lived Greek state centered on the city of Thessalonica, ruled by the Komnenodoukas dynasty of the Despotate of Epirus between 1224 and 1246. For part of that span its rulers held the imperial title and rivaled the Empire of Nicaea and the Second Bulgarian Empire as the strongest power in the region.
Theodore Komnenos Doukas founded the Empire of Thessalonica after capturing the city of Thessalonica in 1224. He was crowned emperor there by Demetrios Chomatenos, the Archbishop of Ohrid, in either 1225 or 1227-28.
The Empire of Thessalonica declined after Theodore Komnenos Doukas was defeated, captured, and later blinded at the Battle of Klokotnitsa against Bulgaria in 1230. The state lost most of its territory in Thrace, Macedonia, and Albania, and Theodore's successor Manuel Komnenos Doukas was reduced to a Bulgarian vassal.
The Empire of Thessalonica was ruled in turn by Theodore Komnenos Doukas from 1224-1230, his brother Manuel Komnenos Doukas from 1230-1237, Theodore's son John Komnenos Doukas from 1237-1244, and John's brother Demetrios Angelos Doukas from 1244-1246.
The Empire of Thessalonica ended in 1246, when Nicaean supporters inside the city opened a gate to let in the army of John III Doukas Vatatzes. Its rulers had already renounced the imperial title in 1242 and been reduced to the rank of Despots of Thessalonica.
The Empire of Nicaea took control of Thessalonica after leading citizens of the city conspired to overthrow ruler Demetrios Angelos Doukas and hand it over. When Vatatzes appeared before the walls in 1246, Nicaean sympathizers opened a gate, and Andronikos Palaiologos became the city's new governor.