Both are charges for keeping a container too long. Demurrage: the full container stays in the terminal past its free days. Detention: you keep the container outside the terminal past its free time.
Free time differs per shipping line and port, and daily rates rise the longer it takes. Delays in customs or missing documents are the usual cause.
How we prevent it: documents checked before arrival, customs filed early, pickup planned, and extra free days negotiated when we expect a delay.