EDIT: Aliens built them there pyramid thingies.

A handful of ancient civilizations built pyramid style structures. You know what that means right? Aliens! That's right, tall white beings from the stars told the smaller brown humans how to build them...err...wait, I just realized how that sounded.

The basis for Aliens helping our ancestors was an interesting thing at one time, however now it has evolved into something stupid and borderline racist. Anthropology and Archaeology are ever changing an evolving fields because new finds open up new possibilities we didn't consider before. Mostly it has to do with how we are realizing humans were more developed than we thought and were extremely quick learners, after all, only the strongest survive.

Pyramids in Egypt
The first pyramids were carved from bedrock and were tiered, not like the smooth looking pyramids you might think of. The shape of the pyramind is said to represent sunbeams coming down from the heavens which they believed how you got into the afterlife in their solar religion. So, they were tombs, and it was a way to show everyone at that time who was boss (Maitland). To cut the limestone for the later pyramids they used copper tools and a hard marerial called Dolerite, also limestone is soft enough to be cut by stone tools in it's bed, after it is removed and exposed to the sun it dries and hardens (NatGeo). Next, the Pharoh would consult a commitee on where to place the pyramid with the help of astronomers to make sure the axis of the pyramids lined up correctly with the constellations. Then a gigantac workforce of about 20-30 thousand people constructed the pyramids using flotation techniques to get the massive stones out of the quarries and about 20 men to drag or push the stones up ramps to place them (National Geographic). You could think of the ramps as scafolding. Many bakeries around the pyramid sites have been excavated which could have produced thousands of loafs of bread to feed this many people (National Geographic).

Quarry for pyramid stones.

"Alien theorists often say that the pyramids couldn’t have been built by such ‘primitive’ ancient people and it’s strange that the pyramids suddenly just appeared out of nowhere" (Maitland). The archaeolgical evidence is enough on it's own to oppose that claim, plus Egyptologist actually have day-to-day records of the goings-on during pyramid building recorded on papyrus paper (Maitland). Some people have a tendency to think that because these things were built a long time ago that there wasn't enough people on to do it, however that's just ignorance. In anthropological terms ancient Egypt was/is considered a state, which means it had a population of hundereds of thousands with a complex social system etc...Also, the poeple of Egypt at that time had different priorities and since they had only one supreme ruler, the Pharoh, he was able to mobilize a project of that magnitude with no questions asked (Maitland).

Another thing, if aliens built the pyramids then why are the cores of the pyramid constructed very sloppy, I thought aliens had super advanced technology? According to Archaeologist Mark Lehner the builders didn't join the stones of the core very accurately and filled the seems with a bunch of mortar. Do you think aliens would be that sloppy? Further proof that men built the pyramids is the fact that the divisions assigned to buld them actually graffitied their divisions name onto the building  such as ‘Friends of Khufu’ and ‘Drunkards of Menkaure’(Maitland), the second one is my favorite.

Mayan Pyramids
The Mayans built "pyramids", that were also made from limestone, to be closer to the gods. They too like the Egyptians were big into astronomy and incorporated that into a religion. Unlike the Egyptian pyramids, most of the Mayan ones had stairs that led up to a temple which housed important members of their society like priests and nobles. The Mayan pyramids were also built to be used as land marks, as most of them were made high enough to protrude out of the jungle, so that Mayans who used trade routes or perhaps even an army could tell were they were in relation to their position. The Mayans, like the Egyptians, were also good record keepers and alot of the information I'm giving you has been translated for their heiroglyphs. - Most of this was provided by Crystal-links.com, but there is other of information on other websites that shares the same. Plus there is more information readily availible in a local library.

Chinese Pyramids
They're burial mounds and tombs. Enough said.

My Theory
I'm big on how environmental factors played/plays a part in past and present cultures. It's obvious that the people who built these things wanted them to be majestic. The human imagination is often inspired by what they see around them, so what in nature is looks like a pyramid, is majestic and possible close to the gods? A mountain. There have been archaeological finds of how mountains were used for a variety with things, but by making your own mountain you can control what it's used for and where to put it. However that's just my theory and I'm still an undergrad, but it's a possibility.

My rant about this alien crap.
When you say that Aliens built all this stuff, you are basically saying humans aren't or weren't smart enough to do any of it. Also there is this idea in western society that people of another color that aren't European couldn't possibly have done anything like that because they didn't have the technology so they had to have help from aliens, when in fact archaeologists are finding that past cultures had more technology than previously thought. Any archaeologist claiming that aliens help build these things and backs it up with a photo of a slab of ancient text is only showing you part of the picture. That slab most likely came from a larger piece and the archaeologist is using out of context to make a name for himself and get money. Lastly, why in the fudge would aliens only show us how to build stone monuments? Wouldn't they rather teach us something else like gee let's say....Inter-stellar travel, or super advanced technology?

Sources:
Maitland, Margret Ph.D. "The Eloquent Peasant » Blog Archive » Why the Aliens Did NOT Build the Pyramids." The Eloquent Peasant. 24 Aug. 2006. Web. 07 Dec. 2011.  I strongly recommednd reading this article.

National Geographic: Egypt Pyramids--Facts, Photos, Diagrams."National Geographic - Inspiring People to Care About the Planet Since 1888. N.p., n.d. Web. 7 Dec. 2011.