APRS is most common used to plot positions of moving objects like cars, boats and planes on a map. As the object moves, the latitude and longitude coordinates are transmitted out on a ham radio frequency in the form of a packet. Other receiving stations pick up these packets, decode the information and plot the position of the object on a map. The refresh rate depends on how often the moving station transmits packets. APRS is also used to transmit weather data.
Weather stations transmit temperature, rainfall, wind speed and direction at timed intervals.
The same software that plots the moving objects on a map also will display the weather information for stations that transmit the data. The National Weather Service also uses APRS to transmit weather bulletins. This is great for APRS stations because they can see where the severe weather is located compared to their present position. Messages can also be sent to other APRS stations. Although the size of the messages are limited to 45 characters, it is a great way to say hi to someone or just get their attention.