There are many glitches, especially on the early launched versions of the game. Many of them have been patched. So, the speedruns are divided into two categories, Glitched and Non-Glitched.

There are many parts which can be skipped, like in Dark Souls, where, without having items (which are usually required, like a double jump), you can make that jump using enemies and bouncing on them. Things like that.