These sites often work along side physical hardware such as wristbands and smartphones and allow you to monitor personal activities and then generate graphs and other information based off that data. For example these sites might allow you to monitor personal fitness by tracking movement or running distances and then help you set goals and calculate improvements required to meet weight loss goals. They might also be used to log eating or drinking habits and perhaps to monitor sleep patterns.
map my fitness
The cost for hardware could be quite large, although apps and sites may be able to run off existing tech you have, such as a smartphone.
Many apps have a small fee although there are also free ones available, the service levels offered can vary and free apps may include adverts and limited functionality.