system
Given the following post, respond as if you're commenting on Reddit:
human
[ELI5]What is an MP3 compared to MP2/1 and MP4, for that matter. 

MP4 is just video? What are the other two!? I'm so confused.
gpt
MP3 is actually short for MPEG 1, Layer 3

MPEG is a standard for audio-video compression, it defines how audio and video information is mixed together and put into a file that can be easily read by your dvd player, your iPod, anything capable of playing a video.  It ensures compatibility. 

When audio and video are stored in this file format, they are stored encoded and compressed following a set of rules. 

For example, an Audio CD can store 74 minutes of music - these discs are raw, uncompressed music. If you'd copy the disc on your computer and keep it exactly the same, you would need 650 MB of disk space. 

When the audio is stored in the MPEG file, the software that saves the file analyzes the music, tries to determine what your ears can't easily notice and removes these parts to make the file use less disk space, then compresses everything as if you would compress a normal file with WinZip or Winrar.  
This way, those 74 minutes of audio would be stored in about 100-150 MB of disk space, instead of 650 MB. 

When the standard was originally invented, they had to come up with a method to remove parts your ear won't notice missing easily and make the audio take less disk space. They did and titled it "Layer 1". This method was somewhat lousy, the audio quality was poor, but this was the best they could do. They had to make a compromise between audio quality and the cost of the products playing the music and video.  Making it very good quality would mean the video and audio players had to use very expensive chips that would also consume a lot of power, so the audio and video players would not be very portable. 
It might sound silly now, but when this first standard was defined, people had 486 computers and probably the first Pentium computers appeared.

A few years later, the technology evolved enough that they could take some of the ideas they had for the first version (but had to abandon due to that compromise) and also thought of better methods to encode the audio and make it use even less space while retaining more quality... in the end the sound was better while it took less disk space than before. This needed much more processing power, better computer chips, but by this time, the chips were cheap and powerful enough to be easy to go ahead with the plan. So they called it Layer 2 ...

More years went by and the chips got even better, more powerful, which made the group behind MPEG to come up with a new set of methods and tricks to compress audio in smaller disk space and keep more audio detail.... and they called it Layer 3.

This MPEG1 Layer 3 method of audio compression became very popular in very short time, because it was made a standard around the time of Napster, so people started to make audio only files and renamed them to have the extension .mp3 instead of the standard .mpg or .mpeg

MPEG-2 is an evolution of this MPEG 1 file format. It was created for DVD, because by now better methods to compress video were possible without using very expensive chips.  The methods used in MPEG-1 to compress video were designed for small resolutions, generally half the video information a DVD had, and had less quality. 

They also worked MPEG-3, which was supposed to be a file format optimized for high definition content (HD) but they decided to give up on it, because they realized the same methods to compress video used for DVD gave good enough results with HD content. 

Now MP4 is actually short for MPEG-4, which is a new standard for storing audio and video mixed together in a file, from the same group that made MPEG, but a much more modern one. In this file, video and audio can be stored using several of those methods, just like with MPEG.  

In MP4, the most preferred methods for compressing audio are AAC (a bit better than mp3)  and mp3, while for the video part the preferred method is called h264. This h264 method of compressing video is much better than the method used in MPEG-2.