Everything that appears to the human eye to be a moving image is in fact a sequence of almost-identical still images, both created and shown very quickly. Typical speeds are tens of images every second. Too slow, and the illusion of movement first becomes intermittent and then is lost altogether. Too fast, and the increasing technical difficulty and requirements are wasted since the human eye can no longer tell a difference.
For the first movies, although the speed tended to vary a little because it was manually provided by the human cameraman, the speed was approximately 16 frames per second. (And each frame or still picture was actually shown twice, as is typical in movie projection.) Later movies in North America standardized on the higher speed of 24 frames per second. (Very old movies typically look jerky because modern equipment is projecting them half again faster than they expected.)
North American television shows moving images at 30 frames per second (also described as 60 interlaced half‑frames per second). (Speed conversion of north american movies for delivery on north american television often involves something called a three-two pulldown.) For easy meshing with their 50‑cycles-per‑second electric power, European (PAL) television (and in many cases also movies) show moving images at 25 frames per second. Formats for moving images on discs (DVD, Blu‑ray) tend to be very similar to television.
Currently high speeds (48 or even 60 frames per second for movies following the North American standard) are being experimented with. They're expected to provide a smoother illusion of movement, more clarity, and a brighter picture. (The brighter picture is particularly important for 3D movies, where each eye gets only half of what a 2D image would provide and so the picture may be noticeably different. The current method of compensating for this problem is to show 3D movies only on special more highly reflective screens.) Picture improvements (increased resolution, higher frame rates, etc.) have downsides too: they require larger distribution media, they often require theaters to make changes to their projection equipment, the increased clarity of images is not always welcomed by audiences, and moviemaking techniques such as makeup and set construction that used to be adequate can easily look fake.
Only a few types of moving images are common in U.S. culture today:
Video Clips are often i] funny, or ii] educational, or iii] propagandistic, or iv] creative, or v] about extremely uncommon topics. Most of them are somewhere between a few minutes and an hour long. They were often shot with traditional video cameras (either consumer or prosumer grade). Usually some care was taken about their production values and they have been edited to some degree. Styles continue to change very rapidly; some more recent video clip trends are: shooting with smartphones rather than traditional video cameras; higher production values without significant expertise or effort simply by using the much smarter software filters and tools that have become available; automatic loop/repeat tying the end of the clip back to the beginning; and very short video clips (somewhat analogous to the 140 character length limit of Tweets). They are by far most readily accessible on the WorldWideWeb, either on individual websites or on public sharing websites (Youtube, Vimeo, Instagram, etc.).
Commercial Recordings are most often encountered on various sorts of broadcast media. They are often intermixed into live programs; their recorded nature is soft-pedalled, not trumpeted or made explicit. Generally aesthetic considerations take a back seat to recording accuracy and clarity. Many of these are Music Videos and are typically only a few minutes long (specifically the length of a popsong). They're generally very highly produced and quite artistic, often containing unusual or striking or fantastic images. The genre is so specific though and their distribution so narrowly targeted that many people seldom if ever see them.
Smartphone Segments by contrast are usually of the same sorts of personal subjects a person would have previously documented with still photographs: close relatives, important events, uncommon happenings, exotic vacations, and so forth. They usually range from only a few seconds to a couple tens of seconds long. They show very little concern for production values (outside of a few generic rules of thumb such as shoot in good light)—spontaneity and authenticity are more important. They are usually only minimally edited ...if they're edited at all. They are often encountered on social media websites (Facebook, Twitter, etc.) or are distributed privately (email, SMS, etc.).
Movies are the main avenue for broad distribution of moving images whose primary focus is artistic. That's what I see most often, and is the main focus of these webpages. Movies are distributed on several different media and so can be seen in movie theaters, on discs, or streaming over the Internet. Each media for distribution and viewing of movies has different benefits and limitations.