Emperor
The first Austrian Emperor was the last Holy Roman Emperor Francis II. In the face of aggressions by Napoleon, Francis feared for the future of the Holy Roman Empire. He wished to maintain his and his family's Imperial status in the event that the Holy Roman Empire should be dissolved, as it indeed was in 1806 when an Austrian-led army suffered a humiliating defeat at the Battle of Austerlitz. After which, the victorious Napoleon proceeded to dismantle the old "Reich" by severing a good portion from the empire and turning it into a separate Confederation of the Rhine. With the size of his imperial realm significantly reduced, Francis II, "Holy Roman Emperor" became Francis I, "Emperor of Austria". The new imperial title may have sounded less prestigious than the old one, but Francis' dynasty continued to rule from Austria and a Habsburg monarch was still an emperor ("Kaiser"), and not just merely a king ("König"), in name.
Kaiser
The Holy Roman Emperors (962–1806) called themselves "Kaiser", combining the imperial title with that of "King of the Romans" (assumed by the designated heir before the imperial coronation); they saw their rule as a continuation of that of the Roman Emperors and used the title derived from the title "Caesar" to reflect their supposed heritage.
In 1806 the Holy Roman Empire was dissolved, but the title of Kaiser was retained by the House of Habsburg, the head of which, beginning in 1804, bore the title of Emperor (Kaiser) of Austria. After 1438, the Habsburgs had provided most of Holy Roman Emperors, so they saw themselves as legitimate heirs to the title they adopted. Despite Habsburg ambitions however, the Austrian Empire could no longer claim to rule over most of Germany, although they did rule over large areas of lands inhabited by non-Germans in addition to Austria. There were four Kaisers of the Austrian Empire who all belonged to the Habsburg dynasty.