Hiking is any outdoor walking activity - along trails or across unmarked wild country.
A hike can be a single day-walk.
Hiking requires some ability to navigate, although there are many well-marked trails available in the more popular areas. In more remote areas where marked paths and trails are hard to come by navigation skills are essential to avoid getting lost.
Sturdy, comfortable and well fitting shoes are a must, and other suitable equipment like warm clothing and waterproof gear are highly recommended.
Multi-day trips carrying camping gear for overnight stops, are generally referred to as backpacking. Questions applicable to both can generally be tagged as just hiking, as hiking is a sub-component of backpacking.