The APRS protocol specification gives a good starting point for describing APRS: "Fundamentally, APRS is a packet communications protocol for disseminating live data to everyone on a network in real time. Its most visual feature is the combination of packet radio with the Global Positioning System (GPS) satellite network, enabling radio amateurs to automatically display the positions of radio stations and other objects on maps on a PC. Other features not directly related to position reporting are supported, such as weather station reporting, direction finding and messaging."
APRS is a good data mode to get started with as it has a very low barrier to entry. You can begin with just some software on your PC, a VHF handheld and two audio cables between them. APRS infrastructure on VHF is common in most areas and so unlike voice communication, you're likely to be able to find a station to send your packets to and confirm your station is working. At first it can seem that APRS is incredibly simple, but later you'll discover it also has a lot of features that make it incredibly powerful as a mode to operate, or as a mode to compliment other modes.