Human magnets are people who can stick various metallic objects to their skin for some time, allegedly due to some kind of "magnetic" properties.
While it's a neat parlour trick, this is nothing paranormal. The "stickiness" is explained with literal stickiness - the natural skin secretions of these people make their skin unusually adhesive. This also explains why "human magnets" can "attract" non-magnetic materials like aluminum and plastic. James Randi has dealt with several of them and suggests using talcum powder to dry the skin before testing. Another possibility is using a thin sheet of (dry) plastic - in both cases, the thin layer of material wouldn't be a problem for a true magnetic field.