The Dreamers are exceptionally well educated and well bred class at the time of the Emperor, and therefore the last Dreamers are dying out during the course of the story of the Prince of Ruin. The Dreamers live a little longer than normal humans, and form the intelligensia of the cities of the Empire.
The Dreamers are essentially a race: the Emperor's Rennaisence initiative took place a few years after he first constructed his capital. The Rennaisence initiative was a plan to form a class that would eventually be bred into a race, through selective breeding and cultural structure based around learning and creativity, he hoped to create a race of elites that could outlast him. He was only partially successful, in that the Dreamers are indeed very superior in terms of education and health as compared to normal humans, but are also very mortal and without the Emperor's protection and sponsorship, unable to continue the lives and stations they once possessed and are instead a scattered diaspora dedicated to the Emperor's memory and recording and interpreting his knowledge. The knowledge is mostly philosophical and whimsical, as after thousands and thousands of years of repetition the Emperor eventually stopped the lectures that educated the Dreamers.
The lectures were informal, and he would simply hold conference with select Dreamers during meals or bathing or whenever he wanted company, and the highest elite Dreamers would accompany him and write down whatever he said. They would then interpret his words according to their previous knowledge of the thousands upon thousands of tomes of his speech, and add that into their summarial notes, updating the scriptures of the Emperor's legacy.
The Prince of Ruin encounters various Dreamers both in his army and elsewhere. The Blood Princess's retinue is composed of many Dreamers, or their issue.