Here is the best explanation of a collider written by 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.