DASH and MP4 are not mutually exclusive - they perform different functions in the video delivery.
In simple terms you can view it like this:
- Camera captures frames - 'raw video'
- The 'raw video' is encoded in some way to store it, generally in a way that balances video size vs the quality. The video is then sometimes refereed to by the encoder used (the codec) - for example if a h.264 codec is used the video may be called a h.264 video.
- The video stream, i.e. all the individual frames that make up the video, is packaged into a container. This container may contain video and audio streams, and it may even have multiple video streams. The video is then often referred to by the container format - for example if our h.264 encoded video above is packaged into an MP4 container it is often referred to as an MP4 video, even though the MP4 'container' may contain several video and audio tracks.
- To improve the quality of video streaming, a video may also use a streaming protocol like MPEG DASH. The theory here is simple: multiple copies of the video are created with different bit rates, and hence different size and quality. Each of these copies is broken up into, for example, 10 second chunks. An index file is created, called a manifest, and a pointer to each video and audio stream is included. A client playing the video, for example a browser, requests each 10 second chunk as it needs it. It chooses which copy of the video it selects the next chunk from depending on the current network conditions. This means if the network is good it can switch to higher quality copy for the next chunk and if there is a problem it can switch down to a lower quality chunk. If we take our example video encoded by h.264 and put into a MP4 container, we can now package it using DASH streaming format. A video packaged like this is often referred to as a DASH video.
The above is a simplified overview, but it hopefully highlights that your videos may be actually MP4 and DASH, and in fact commonly are.
As an additional note, different devices may support different codecs (and even codec profiles), packaging formats and streaming formats - for example iOS devices tend to support HLS rather than DASH at the time of writing. This changes frequently as devices and standards evolve and is one of the reasons it can be tricky to find a single format that will play on all devices and clients - for this reason servers often will provide the same video in multiple codec and streaming formats to support as many devices and clients as possible.
