Why DMR, D-STAR and System Fusion are the three common amateur digital voice systems, mostly used on VHF/UHF repeaters. Each digitizes speech with a vocoder (typically AMBE/AMBE+2) and sends the resulting bit stream, usually with call sign and other data riding along. They are not interoperable with each other, but all three carry voice rather than keyboard text.
Watch out FT8, FT4 and FST4 are tempting because they are the best known modern digital modes, but they are weak-signal data modes that exchange only tiny structured messages, no voice. Winlink and the PACTOR modes carry email and file data, WSPR is a beacon mode, MFSK16 is text chat, and EasyPAL sends digital images.
Voice modes are the repeater trio: DMR, D-STAR, Fusion. Anything with numbers in the name (FT8, PACTOR II) is data.