"When the Greek historian Herodotus visited Egypt in the fifth century B.C., he was told by his guides that 100,000 workers had labored for 20 years to build Khufu's pyramid." Source: Dr. Zahi Hawass, former Egyptian Minister of State for Antiquities Affairs.
"Even 20,000 workers, a number closer to recent estimates, is comparable to the populations of large cities in the Near East during the third millennium B.C."
Technically, most of these workers weren't slaves. "Ordinary Egyptian citizens built the pyramids, some working as conscripts on a rotating basis, others as full-time employees," according to National Geographic (2001). Note, however, that the people who had been conscripted for this heavy labor, even if it was on a rotating basis, weren't exactly what we would call free citizens since they were periodically drafted into a form of national service. In the next video (2015), Dr. Zawi Hawass claims that the pyramids were built "through the love of the people," but I really doubt if this applied to the conscripts.
Building the pyramids of Giza required a series of massive, long-term construction projects. "Hawass and Lehner estimate that the feat—quarrying, transporting, and fashioning the seven million cubic yards (5,351,885 cubic meters) of stone for the three pyramids and adjoining structures—was accomplished with a workforce of only 20,000 to 30,000 men. Each pyramid complex (a grouping of pyramid, temples, and tombs) was started when a pharaoh assumed the throne and stopped when he died. Thus the Giza monuments, which were constructed during the 4th-dynasty reigns of Kings Khufu, Khafre, and Menkaure (about 2550 to 2470 B.C.), required some 80 years to build." --Ibid.
I'm guessing that most of the conscripted laborers were probably used to transport the millions of stones used to build the pyramids. This was no easy task. In the case of the Great Pyramid, most of the stones weigh somewhere between one to ten tons with an average weight of around 2.5 tons, while some of the blocks used in the king's burial chamber weigh up to 50 tons, according to Building The Great Pyramid. The calculations used here estimate that "342 stones have to be moved daily (working during 365 days a year) or 431 stones daily (working during 290 days a year)." That comes to around 30-50 stones per hour depending on the length of the work day.
How hard was this work? It must've been exhausting, to say the least, to labor under these assembly-line conditions in the hot Egyptian sun all day, but the work couldn't have been too backbreaking or the builders would have died like flies and the pyramids would never have been constructed. The sheer number of workers involved would have made the job much easier as well. The amount of manpower available is frequently overlooked in all the speculative theories about how the pyramids were constructed.
"It has been estimated that a ratio of two men per ton would be required for moving loads over flat surfaces; nine men per ton would be required for moving loads up a 9° slope. Practical experiments moving loads on a sledge over a lubricated track have shown that one man could pull one ton. Thus, the 1,000 ton colossus of Ramesses II could have been moved by 1,000 men (or 200 oxen)." Source: "Moving Large Objects," Catchpenny Mysteries Of Ancient Egypt.
These are minimal requirements, I think. In order to meet their production schedule, I would imagine that the Egyptians used very large work gangs to move the blocks. Assuming a total work force of 20,000, with maybe half of them involved in quarrying, finishing, fitting, water transport, etc. (my guesswork), that leaves 10,000 men for hauling the stones into position. I have no idea how many workers were used to move a single stone, but teams of 50 or 100 men wouldn't have much trouble dragging a two-ton block up a ramp and the next team would probably be right behind them. And so it would go, all day long, day after day.
If this view is accurate, then the pyramids were built using sophisticated stoneworking techniques and the combined muscle power of large numbers of workers. I don't know if this is how it actually happened, but it's the simplest explanation. It doesn't require any secret "lost technology" or the assistance of alien tractor beams or any of the other theories that have been floating around ever since the pyramids were first seen by foreign travelers.
As for the workers themselves, they probably didn't have it too bad by ancient standards. Many of them were seasonal conscripts, granted, but they weren't slaves in forced-labor death camps. They were working on huge construction projects and they must've been well-fed and they probably had reasonable accommodations and decent medical care. They were probably manual workers and farmers in their private lives and their periodic service at the pyramid sites may actually have been a step up in their living conditions.