Storage media

Digital audio formats can be stored on any digital storage media. The first mass-market digital audio format was the audio CD, which standardized a storage media as well as a digital audio encoding format. The encoding format (uncompressed WAV files sampled at 44.1KHz) was also used to store digital audio on other devices, but other encoding formats were also developed.

Digital storage media includes

CDs - for which write once and and write many formats were added to the original read-only manufactured ("pressed") format.
Data CDs - to hold non-audio digital storage, but can also be used to hold digital audio in other formats than the original.
DVDs - basically a higher capacity CD, and for which a double-sided format was eventually added. There are several standards for storing digital audio on DVDs, but none have become popular in the mass-market. DVDs are mostly used for digital video formats, although that is giving way to streaming video on the internet.
Magnetic disks - mostly known as "hard disks" in contrast to "floppy disks". While floppy disks could be used to store digital audio, it was seldom done because of their limited capacity.
Flash drives - with various capacities, sizes, and connectors - were initially used for general data storage, but could be used for digital audio as well. Over time, some particular formats have become more popular than many others
- SD card
- micro SD card
- USB drives, also known in some parts as thumb drives
Most of the flash drives required unique connectors for their size and shape, and that is true of SD and micro SD cards as well, although the latter can be placed in a holder and used in an SD card connector. USB drives use a "standard" connector, which sounds more general, and is, but over time there are also several variations of the USB standard, and some of them sport different connectors, so it isn't quite as universal as originally envisioned.

The flash media has continued to gain in capacity and speed, and shrink in size and cost, so is taking over much of the digital data storage market. Because of that trend, and the far smaller interfaces required to read flash media compared to CDs, it is even taking over the digital audio storage market, and newer vehicles do not reserve space for CD players, but rather have SD card or USB interfaces for user digital audio, and generally also Bluetooth interfaces so audio can be played from smartphones.

Audio formats

There are many different audio formats. For details on most of them, see this Wikipedia article. Uncompressed formats include WAV and AIFF. Compressed formats come in two flavors: lossless and lossy. The article discusses all three kinds. Some formats are were created by particular manufacturers, others via standards bodies.

By far, the most universally supported formats are CD audio (lossless and uncompressed) and MP3 (using parameterized lossy compression). While other formats may be "default" or "preferred" on different systems, and by different people, and some media players can play quite a variety of different formats, every modern (and many obsolete) computer, smartphone, automobile digital audio interface, and standalone player I've ever heard of supports at least one of these formats.

Among CD players, there are three varieties: CD audio only, CD audio + MP3, and CD audio + MP3 + some number of other formats.

Among standalone digital players, there are two varieties of modern interest: MP3 only, and MP3 + some number of other formats.

An MP3 file compressed to 128Kbps bitrate (the bitrate is the primary control over the compression factor) is indistinguishable to most people from the uncompressed audio, and about one-tenth the size. This is the format I use for compressed audio files.