Pyramids are structures that appear to be great triangles. That's because they are formed with triangular surfaces. A pyramid may have 4 or 5 sides, since the base can be either square or triangular. The most familiar pyramids have a square base and 4 triangular sides that come together in a point at the top. Other pyramids have a flat top as roof.
The pyramids of Egypt were used as funerary structures. Other pyramids are connected with temples. There are also Mesoamerican and Mesopotamian pyramids.
Pyramids may have smooth surfaces or they may be stepped.
Pyramids are remarkably sturdy as can be told by their great age.
Of the 7 wonders of the ancient world, one survives, one of the Egyptian pyramids -- at Giza.