A showing is a scheduled, private tour of a listed home for a specific buyer, usually arranged through their agent. Only that buyer and agent attend, which is the main difference from an open house, where anyone can walk in during a set window.
The two capture leads differently. A showing is already a qualified, represented buyer, so there is little to capture. An open house is a stream of mostly unrepresented visitors, which is why the sign-in and follow-up matter so much there.