Super Mario 64 is a Mario game that Super Mario 64 machinimists use to make their videos.
You can do many things to Super Mario 64. You can make it completely different, retexture it, or just play it.
People like Skelux, Lugmillord and MarioGame2222 have made/are making hacks of Super Mario 64 that make it look completely different. Often, they rename this hack.
Skelux has made Super Mario: Star Road and is currently working on Super Mario: Star Road 2. MarioMario54321 used a retextured version of Star Road to use for the Fourth Dimension, and LuigiLuigi573 wants to use Star Road 2 for a series when it comes out. When LL573 asked Skelux about the release date of SM:SR2, Skelux simply said, "A long time".
Lugmillord created Super Mario 74, which MarioMario54321 uses for the Moon World (but retextured) and The2Hunters used for the Third Dimensional World. It is unknown if he is making any more hacks at the moment.
MarioGame2222 is creating Super Mario 64: Star Zone. The estimated release time of SM64:SZ is December of 2013.
Some machinimists retexture hacks to use for their videos. MarioMario54321 retextured Super Mario: Star Road to use for the Fourth Dimension and Super Mario 74 to use for the Moon World. In most of his videos before 2012, MM54321 just retextured Super Mario 64 without any other hacks. The2Hunters used a texture pack made by dshaynie for the Third Dimensional World. MarioMario761 made a texture pack for the Multi-Dimensional Road. This list could go on and on for a while, but let's go to the next section.
Some people just use the plain Mushroom Kingdom for their videos, like a quest to collect 15 special stars, or something like that.
As stated before, Super Mario 64 machinimists use SM64 for their videos. They can simply have Mario running around collecting stars or be in an entirely different hack with completely different colors compared to Mario. It all depends on the player.