Moving Through a Creature's Space
Source Player Core pg. 414
You can move through the space of a willing creature. If you want to move through an unwilling creature's space, you can Tumble Through it. You can't end your turn in a square occupied by another creature, though you can end a move action in its square provided that you immediately use another move action to leave that square. If two creatures end up in the same square by accident, the GM determines which one is forced out of the square (or whether one falls prone).
Prone and Incapacitated Creatures
Source Player Core pg. 414
You can share a space with a prone creature if that creature is willing, unconscious, or dead and if it's your size or smaller. The GM might allow you to climb atop the corpse or unconscious body of a larger creature in some situations. A prone creature can't stand up while someone else occupies its space, but it can Crawl to a space where it's able to stand, or it can attempt to Shove the other creature out of the way.
Creatures of Different Sizes
Source Player Core pg. 414
In most cases, you can move through the space of a creature three sizes larger than you or larger. This means a Medium creature can move through the space of a Gargantuan creature and a Small creature can move through the space of a Huge or Gargantuan creature. Likewise, a bigger creature can move through the space of a creature three sizes smaller than itself or smaller. You still can't end your movement in a space occupied by a creature.
Tiny creatures are an exception, just like with sharing a space. They can move through creatures' spaces and can even end their movement there. Similarly, other creatures can move through and end their movement in a Tiny creature's space.
Tiny creatures are an exception, just like with sharing a space. They can move through creatures' spaces and can even end their movement there. Similarly, other creatures can move through and end their movement in a Tiny creature's space.
Objects
Source Player Core pg. 414
Because objects aren't as mobile as creatures are, they're more likely to fill a space. This means you can't always move through their spaces like you might move through a space occupied by a creature. You might be able to occupy the same square as a statue of your size, but not a wide column. The GM determines whether you can move into an object's square normally, whether special rules apply, or if you're unable to move into the square at all.