Rules:
A set consists of three cards which satisfy all of these conditions:

They all have the same number, or they have three different numbers.
They all have the same symbol, or they have three different symbols.
They all have the same shading, or they have three different shadings.
They all have the same color, or they have three different colors.

The rules of Set are summarized by: If you can sort a group of three cards into "Two of ____ and one of _____," then it is not a set.

For example, these three cards form a set:

One red striped triangle
Two red solid triangles
Three red open triangles

Given any two cards from the deck, there will be one and only one other card that forms a set with them.