Everybody knows about confounder but what is a collider?

Here is the best explanation of a collider written Julia Rohrer at www.the100.ci

Whenever X1 (conscientiousness) and X2 (intelligence) both cause Y (college attendance) in some manner, conditioning on Y will bias the relationship between X1 and X2 and potentially introduce a spurious association (or hide an existing link between X1 and X2, or exaggerate an existing link, or reverse the direction of the association…)

The cartoon makes it even clearer – confounder act on exposure and outcome, while collider condition on exposure and outcome.